Blue Prussian Pea
The Blue Prussian pea has been known by a variety of synonyms. The seeds were (are?) not really blue, let alone "Prussian blue". Rather, they had green cotyledons covered by a whitish, translucent seed-coat. The most striking qualities, aside from color: half-dwarf plants, branched, usually with two pods per node.
Ellis: The Modern Husbandman (1744)
... blue Union, &c are all of them of so tender a Nature, that, if they are thoroughly pinched and nipped by a very hard Frost at Root and Stalk, they can never recover a right Growth again, but will shew their Sickness and Decay by their red and withering Heads.
Doctor Zimmermann's conversations with the late King of Prussia (1791)
I learned too that no idea could be formed of the excess which the king allowed himself in his food; that his cooks were obliged to season all his dishes in such a manner, as was enough to destroy his stomach; that those which were most difficult of digestion were his greatest favourites; that he was passionately fond of Prussian peas, which are undoubtedly the hardest in the world, and would consequently be, considered as coarse even in Lower Saxony and Westphalia;
Young: Gleanings from Books on Agriculture and Gardening (1802)
12. Prussian Pea. The seed when dry a light green.
Knight: Influence of the Pollen, in cross breeding,
upon the Colour of the Seed-coals of Plants, and the Qualities of their Fruits. (1824)
The cotyledons of the Blue Prussian Pea, which was the subject of Mr. GOSS'S experiments, are, on the contrary, blue; and the colour of these being perceptible through the semi-transparent seed-coats, occasioned those to appear blue, though they are really white;
Horticultural Register 3: 208 (1834)
7. Blue Prussian.—Syn. Nain Royal—Nain vert petite.—Fine long podded dwarf. Dwarf blue prussian. Blue union. Early dutch green. Early green. Green prussian.
This is too well known to need any lengthened description, I shall therefore merely say that it is a most abundant bearer, and of excellent quality. Grows between three and four feet high. Seed light blue, middling size.
Magazine of Horticulture 2: 430-431 (1836)
33. Blue Prussians. French Synonymes: Nain vert petit, nain royal, gros vert de Prusse. English Synonymes: Dwarf blue Prussian, royal Prussian blue, fine long-podded dwarf, Prussian prolific, early Dutch green, green Prussian.—About 3 1/2 feet high, and of strong growth. Pods long and rather round, containing 8 peas. This is so well known that it is quite useless for me to say any thing about its good qualities. It is undoubtedly the best for summer use, and one of the greatest bearers.
The Agriculturist's Manual p. 73 (1836)
13. White Prussian Pea. Pods generally in pairs, two and a half inches long, from one
19. Blue Prussian. Pods almost always in pairs, similar in size and shape to those of the White Prussian (No. 13), rather larger, and more compressed, colour bluish-green; in flower 16th June, ripe 18th August; height three to four feet; very prolific.
The green straw, pods, &c. is in this, and all the other blue pease, of a darker colour than in the white ones; the colour of the straw varying (although not to the same extent) according to the colour of the ripe seed.
The Principles of Agriculture, 2: 459 (1844)
Albrecht Daniel Thaer
The grey Prussian pea, which is large and angular in shape, and bears a violet coloured flower, is said not to bear change of climate, and to degenerate. In Line and Weser, a grey pea, bearing violet coloured flowers, is cultivated; but it is grown almost exclusively as fodder for cattle, and considered to be unfit for the use of man on account of its unpleasant taste. This is probably a variety of the Prussian pea.
Annals of Horticulture (1847)
p. 124: The Blue Prussian Pea grows strongly, about four feet high, producing seven or eight peas in a pod. A very highly esteemed sort, and one of the best for summer use, and is perhaps the most prolific of all.
p. 126: The Blue Prussian Pea, syn. Royal Prussian Blue, Prussian Prolific, Green Prussian, Early Dutch Green
The Rural Cyclopedia 3: 763 (1851)
The blue Prussian pea, or Prussian blue pea, is a well known and excellent variety; and though at one time sadly degenerated and almost extinct, is now thoroughly restored and quite abundant. It produces a full and remarkably regular crop; it is hardy and accommodating enough to be well suited to the field; and, being of medium earliness, it serves well in garden series to come between the early and the late varieties. Its haulm is commonly about 3 or 3 1/2 feet high; its pods are similar in shape to those of the white Prussian, but rather larger and more compressed, and of a bluish green colour; and its seeds are bluish grey, and, when dressed, are beautifully green and very tender. The whole plant of this variety, as well as more or less the whole plant of every other blue or greenish seeded variety, has a darker shade of verdure than that of almost any white-seeded variety.
McIntosh: The Book of the Garden (1855)
13. Blue Prussian. — A well-known excellent pea. We notice it here merely to give the synonymes — early Dutch green, fine long-podded dwarf, dwarf blue Prussian, royal Prussian blue, Prussian prolific, and green Prussian.
Darwin: Variations (1868)
The stems of the Prussian pea are much branched. ... the moderately tall Blue Prussian, have leaves about two-thirds of the size of the tall kind.
Journal of Horticulture (1872)
Blue Prussian.—Plant of a vigorous but not robust habit of growth, with a single stem about 3 feet high, and which is sometimes branching. The pods are generally produced in pairs, but are also sometimes single, and vary from twelve to sixteen on each plant. They are from 2 1/2 to 3 inches long, three-quarters of an inch wide, somewhat curved, and rather broader towards the point, where they terminate abruptly. They contain about seven peas, which are four-tenths of an inch long, seven-twentieths wide, and about the same in thickness, and compressed on the sides from being so close together. The ripe seed is blue.
This is a very old and popular variety, extensively used in field culture and market gardens on account of its great fertility —a character which it maintains superior to any of the other blue Peas, most, and indeed all, of which during the past season have exhibited much less hardy constitutions.
Documents of the Assembly of the State of New York 4: 258-259 (1884)
No. 47. Blue Prussian.
Plant three to four feet high; foliage rather pale-green, very slightly glaucous; stem medium or large, sometimes branched at the base and above; nodes rarely exceeding three inches apart; peduncles one-half inch to one and a half inches long; pods slightly paler than the foliage, often in pairs, sometimes straight, usually slightly recurved, plump, two to three inches long, five-eighths of an inch wide, very blunt at the apex when fully developed; peas four to eight in a pod, whitish-green, roundish or slightly oblong, much compressed when full grown, about seven-sixteenths of an inch in longest diameter; seeds greenish-white, with blotches of very dull pale-green, somewhat indented, nearly three-eighths of an inch in diameter, radical rather distinct. An ounce contained ninety seeds.
Not prolific, very late, maturing its crop very slowly.
Though generally considered as a field variety, this is sometimes grown as a garden pea. "It is unquestionably the parent of the Blue Imperial and all like varieties," and possibly also of the Knight's Marrows. Known in England prior to 1828.
Vilmorin-Andrieux Et Cie: The Vegetable Garden (1885)
Blue Prussian.—A half-dwarf kind, growing from about 2 1/2 to over 3 ft. high. Pods generally in pairs, seldom solitary, almost straight, and square at the end; peas large, round, very green, becoming bluish when ripe. This is one of the kinds which are most extensively grown by market gardeners.