Back Loop: When you are holding your crochet work, look at the top of the work. The loops on the upper edge, on the side of the work facing you, are the front loops. The other loops behind those are the back loops.
According to The Craft Yarn Council of America's web site at yarnstandards.com, "Back Loop" is the standard definition for BL when it is used in contemporary crochet patterns.
Block: This definition of the abbreviation "Bl" can sometimes be found in vintage crochet pattern books, particularly books from the 1940s.
Bobble: The more standard abbreviation for "Bobble" is "bo," but you might occasionally find it abbreviated as "bl."
If you've encountered this abbreviation in a pattern, and you're trying to figure out which of these meanings is intended, use the context to help you figure it out.
- If the pattern designer intended for you to crochet in the back loop of a stitch, the pattern will usually include language that says something like "Work in BL of stitch."
- If the designer intended you to crochet a block, the pattern will usually include some sort of chart, diagram, or instructions for working it.
- If the designer intended for you to crochet a bobble stitch, directions for the bobble will usually be specified.