D&D 5E - Bless Spell and Total Cover / Line of Sight
Interesting... Then can you bless a target you can't see? What is the precise purpose of having a clear path to the target? I would think that would be so you can see the target, but since the Bless spell does not specify the target must be one you can see (as opposed to a spell such as Bane, which specifies you must be able to see the target), I would interpret the clear path to target to not be necessary.
Of course, at this point it all becomes about interpretation; there is no right or wrong answer, and each DM can rule it as they see (or don't see ) fit.
