Yes, yes he does! (Yeugh.) Not habitually, just ritually. So he’s not like a vampire just chugging the blood of enemies. (Granted, he is kind of a crazy guy.) The rest of what Herodotus had to say about Scythians drinking blood is here:
“As regards war, the Scythian custom is for every man to drink the blood of the first man he kills. ... They have a special way of dealing with the actual skulls - not with all of them, but only those of their worst enemies: they saw off the part below the eyebrows, and after cleaning out what remains, stretch a piece of rawhide round it on the outside. If a man is poor, he is content with that, but a rich man goes further and gilds the inside of the skull as well.”
Of course, Herodotus wrote that the Scythians had a lot of bizarre customs (to Greeks). I don’t actually think that the blood drinking and skull cup thing is entirely untrue, though. Many cultures throughout history and across the world have made cups or decorative items out of real human skulls. And of course, blood has long been an important ritual aspect for many societies. Idanthrysus would only take a sippy sip from very important rivals that he’s defeated. He might lick a bloody sword just to scare someone else. Although he doesn’t do it very often, he doesn’t mind other people thinking he does it a lot, since it’s good PR if you want everyone to think you’re terrifying and badass.