Easily mistaken for : blue versions of Scarlet Pimpernel * (Anagallis arvensis ssp. arvensis), which usually, but not always, have scarlet coloured petals. The difference between the two is tiny: the edges of the petals of Blue Pimpernel are smoother with fewer fine teeth, and the glandular hairs, when examined under a microscope, have four sections rather than three and the gland at the tip of the glandular hair is ovaloid rather than spherical.
*Scarlet Pimpernel can not only sometimes have blue petals, but otherwise pink or white petals. These polymorphisms are not unusual, and in the case of Scarlet Pimpernel seem to occur when the plant is under heavier sunlight, such as may occur more often in the South of England, but much more often in the Mediterranean, where almost all Scarlet Pimpernels are of the blue polymorphism.
Just as with Scarlet Pimpernel, the leaves of Blue Pimpernel are arrow-head in shape, attached directly to the stem without stalks in opposite pairs, with the two branches peeling off just above the leaf junctions at 45 degrees upwards and aligned with the leaves. The stems are square, and adjacent pairs of leaves are set at right-angles to each other. Blue Pimpernel is much rarer than Scarlet Pimpernel.
Probably like Scarlet Pimpernel it will be poisonous containing Arvenins and Cucurbaticins such as Arvenin I and Cucurbaticin B.
The deep-blue coloration of the petals is due to the presence of blue-purple crystals of Malvidin-3-Rhamnoside, an anthocyanin, which it stores at a pH of about 4.6 (slightly acidic) in order to keep stable.
The centre of the flower has the same colour of purple as does Scarlet Pimpernel, so it is possible that it also contains the same purple pigment as that: Oenin, chemically Malvidin-3-O-β-Glucopyranoside.
See Scarlet Pimpernel for more information on the anthocyanins within both sub-species.