Lewis & Short

2. blatta, ae, f.: blatta θρόμβος αἵματος,

  1. I. a clot of blood, Gloss.
    Hence,
  2. II. (Access. form blattea, Ven. Carm. 2, 3, 19.) Purple (similar in color to flowing blood; cf. Salmas. Vop. Aur. 46, and Plin. 9, 38, 62, § 135; late Lat.): purpura, quae blatta, vel oxyblatta, vel hyacinthina dicitur, Cod. Th. 4, 40, 1: serica, ib. 10, 20, 18: blattam Tyrus defert, Sid. Carm. 5, 48; Lampr. Elag. 33; Cassiod. Var. Ep. 1, 2.