A bitter taste in my view should be a warning sign of an nBOMe, or something else that is not LSD.
You should really be testing your acid anyway unless you really trust the source (and I don't mean you just got it from a trusted friend who swears it's LSD - most people don't know what they're taking). However if you just can't be bothered, don't know how and can't be bothered to find out, or for some other reason are just dead set on not testing, and aren't too concerned with taking an unknown hallucinogen as long as you get some kind of trip from it... then just err on the side of caution and don't drop like a 10 strip your first time or anything stupid like that (just take ONE at most).
I've never personally had acid that tasted of anything other than paper but apparently some people have, and maybe some people have more sensitive taste buds than I. Keep in mind that it's typically going to be a very small amount of ink on the blotter - if it's just a VERY, VERY slight bitterness then maybe it's OK... if it's numbing, tingling, or you get ANY other weird sensations other than just taste then I would say there's a high probability that you do not have real acid.