Where does this come from? I don't think Bela Lugosi ever said "bleh" or "blah", yet it's part of the stereotypical Dracula schtick. My son and I are building the Revell Dracula, and for fun are talking like Bela as Drac. Invariably, "bleh" just instinctively comes out.
It's probably just someone, years ago, picking at Bela's thick Hungarian accent. If Dracula (or any Drac-like vampire) doesn't have said accent, he ain't a vampire to most folks! Especially in spoofs! Even Sesame Street's Count has the accent!
It's probably like Superman changing in a phone booth, or Kirk saying "Beam me up, Scotty". It never happened much, or in the case of TOS, not at all, but somehow became a core part of the mythology of each property.