Board Thread:General Discussion/@comment-25266931-20161223190810/@comment-26397825-20161227000016

Could be both, to be honest; blood for controlling coming out the bootom where Ruby severed it, and poison out the tip from the gland, which he used to scratch Qrow. The purpleness of the blood...to show that it's not human? I dunno. What colour is scorpion blood?

The other option is that it is his semblance, which turns his blood poisonous and purple to show that his semblance is active, and he can secrete it through his stinger for attacks.