What is "blue roach"

Rudd \Rudd\, n. [See Rud, n.] (Zo["o]l.) A fresh-water European fish of the Carp family ( Leuciscus erythrophthalmus). It is about the size and shape of the roach, but it has the dorsal fin farther back, a stouter body, and red irises. Called also redeye, roud, finscale, and shallow. A blue variety is called azurine, or blue roach.