Oh my god, I'm so relieved to see I'm not the only one relatively bothered by the "BL" labels xD
I knew once I started seeing them on a regular basis, it was a result of SmackJeeves/Inkblazers comics immigrating over here.
For anyone who does BL comics or is thinking of making one, you really don't need that small BL label in the corner. Most people don't really care, and can guess pretty easily from the thumbnails or the descriptions that it's going to be BL. Just mark any sexual/adult sensitive pages NSFW.
BL is just a genre, and people here on Tapastic are generally accepting of that specific genre, so you don't need to put it on the thumbnail. It would be like if gag comic creators put "gag" in their thumbnails. Let your art speak for itself!
I also agree with an earlier statement made here about how the label somewhat "cheapens" the comic. It's like, yeah, BL-lovers is the main demographic, but maybe some people just want to read it for the content/humor/characters/whatever it is. Putting "BL" on it, to me, just screams "PRETTY BOYS GOING AT IT" and that shouldn't be the only thing you're trying to sell with your comic. It may not be your intention, but that seems to be the general consensus of people seeing your thumbnail, so don't put your series at risk for it.