Just my two cents although I don't have a suggested translation: in my opinion, "block" in this sense is not at all slang. It's a perfectly normal word that can be used in any context, even a formal one. However, the size of a block varies, so it is not a very precise expression. It is used because it gives people a concrete idea of a certain distance. A normal healthy person has no trouble walking a couple of blocks even at a brisk pace.It's not defined; it's slang. It should never be used in legal documents or surveys.
In your sentence it means something like:
تعرف تمسي مسافة شارعين؟