I agree with keii's line of thinking (at least with the very niche part). When I see "BL" on the thumb I immediately pile the series into the list of yet another wish fulfillment comic between two ~hot boiz~ written by a 16 year old girl who fetishizes homosexual relationships and has no concept of how real relationships actually work (more often than not paired with scary conceptions about how male rape is "hot" and OK). Also I'm assuming just by the BL on the thumb that there is no actual plot and is just two men kissing because reasons.
Some people like these comics (well, more than some, SmackJeeves is inundated with it), and that's fine, but I stay away from them like the plague.
Edit: I should say that I have never fully read any BL comics, mostly because I despise romance. But skimming pages and some reviews on certain popular BL comics horrifies me with the themes those particular comics are presenting. Which is why I now have the bias against anything with a "BL" in its thumbnail since the only BL I have been exposed to is that generic seme/uke shit with no plot and is just there for fap material. I would assume anything with an actual plot and isn't just fap material that also happens to be a homosexual relationship is just a romance and not BL.