"In this case, Psammenitos got his reward for devising evil, for he was caught tempting Egyptians to rebel. As soon as he was discovered by Cambyses, he drank bull's blood and died immediately. That's how he died."Νῦν δὲ μηχανώμενος κακὰ ὁ Ψαμμήνιτος ἔλαβε τὸν μισθόν· ἀπιστὰς γὰρ Αἰγυπτίους ἥλω· ἐπείτε δὲ ἐπάιστος ἐγένετο ὑπὸ Καμβύσεω, αἷμα ταύρου πιὼν ἀπέθανε παραχρῆμα. Οὕτω δὴ οὗτος ἐτελεύτησε.
Recently, I went through a pile of old Finnish medical journals. One of them has a "Columna latina", which is a regularly appearing column about the ancient world (both Latin and Greek, despite the name). There's rarely anything interesting there, but one article caught my attention and brought to my mind this passage in Herodotus. It suggested that the reason why bull's blood was used as a deadly poison in antiquity to kill e.g. Midas (the article had some other passage in mind than this) was because after a specific treatment, it would cause botulism. No source was sited, but it looked plausible to me: the blood is kept for some time in sheep's intestine, which contains anaerobic bacteria, including clostridium botulinum. This bacterium produces botulinum toxin, which is the most potent toxin known to mankind. Very small doses are sufficient to paralyse muscles, including muscles of breathing. (The toxin is also used medicinally in very tiny doses in injected drugs like Botox).
The commentaries are not much help here, they all basically repeat the idea that there was some sort of irrational belief that bull's blood was poisonous. To me, this looks too specific and widespread to be just a superstition. Although I'm not a microbiologist, the suggested procedure would be just the sort of situation where clostridium botulinum would thrive; the real question is whether it produces the toxin consistently enough. If there's a 10 % chance, it's enough to prevent any sane person from willingly consuming it, but it wouldn't be enough to make it an efficient poison.