I think what constitutes as a BL/GL comic/novel is that a relationship, or the progression of a relationship, between two people (usually protagonists) of the same sex is the main focus and is related to the main conflict somehow, either as a catalyst or the cause of said conflict or whatever.
I'm creating an LGBT comic, but the relationship between the two protagonists and the progression of it is not the main focus, so it's one of the reasons why I didn't label it as BL.
But yea. As mong2 said you should probably note that the comic has LGBT themes, because if you don't chances are you'll get comments along the lines of "I hope there isn't any gay relationships I've been looking for a straight one"