Cyanidin is one of the six Anthocyanin subsets, and its glucoside Cyanidin-3-Glucoside (C3G) has been garnering attention for its ability to decrease blood glucose levels, and its ability to not hinder muscle protein synthesis at the same time (downstream of AMPK activation).
Cyanidins are a sub-category of the dark pigments found in blue-black fruits and berries as well as some purple vegetables known as Anthocyanins. Cyanidins can be seen as the most pharmaceutically effective anthocyanin subcomponent as they seem to have the greatest uptake rate, the least decay, and the most clinical significance out of all anthocyanins.
It has various effects in cells, most of which can be described as being anti-diabetic and possibly slightly benefit other parameters associated with 'metabolic syndrome' (anti-inflammatory, anti-oxidant, etc.)
It does have some problems with absorption though, so the results you see in in vitro (in laboratory) studies may not apply to when it is consumed. Its bioavailability (percent absorbed) is a concern, and human intervention studies important for this reason.
Benefits have been seen with blood sugar reductions in the range of 150mg/kg bodyweight, a dose well above what is achieved through foods.
It is possible long-term benefits may be seen with a lower dose of cyanidin compounds through foods, due to the many correlations of plant intake and health; however, causation has not been given to cyanidins as of yet.
Additionally, it may be possible to increase bioavailability and thus lower the needed dose by inhibiting P450 enzymes (similar to curcumin being potentiated by piperine). However, this is for the most part currently an untested hypothesis.