It does kind of bother me that canon treats Blackbeard's flag as special and famous when the fact of the matter is that the design of Blackbeard's flag isn't really talked about in contemporary sources and it isn't until, like, a century later that his "famous" flag is actually attributed to him, but then, the very fact that people believe it is his flag would sort of retroactively make it his. It's stuff like that which makes it hard to tell when something is a failure of research on the part of the creators and when the creators are leaning into popular beliefs like that because they're popular enough to affect the Servant's manifestation.
On the other hand, I don't like that Blackbeard is so "modern," too. I might not be a huge fan of One Piece myself, but just what I know about the Blackbeard in that series is probably a lot closer to the historical man than what we got in FGO, and it's not even supposed to be the historical pirate, just a character inspired by him.
If I had been designing the character, he would have been menacing. Terrifying in his appetites and mannerisms, to the point that even Taylor might wind up a little spooked.
Unfortunately, I have to work at least somewhat with his canon personality. I hope the depiction I eventually settled on will manage to feel a little less goofy and ridiculous than he did in canon Okeanos.