Is this the smallest known rebus puzzle, and what does it mean?
The answer to the puzzle is "Blueprint" but I doubt it's the smallest.
We would need to clarify what is meant by "smallest". If you mean the smallest number of bytes required to store the puzzle electronically, then the answer is no, this is not the smallest puzzle. A4ID (foreign aid) (4 bytes) has less letters and color doesn't matter so there's no need to store the information. R+R (summer) is shorter, still.
Side note: We could get cute with it and say "small print" is the smallest rebus: