Boron is scarce; it's one of the x-process elements, and those inherently have low abundance (although the boron-to-carbon ratio in the Earth's crust is enhanced over the solar system ratio, I believe.)

Boron nitride is weirdly analogous (indeed, isoelectronic) to carbon in that there's a graphite-like form (hexagonal BN) and a diamond-like form (cubic BN).

One thing that would hold back BN life is that by themselves B and N form more stable compounds than carbon does. Nitrogen in particular forms molecular nitrogen, which is annoyingly tightly bound.