Build-Deps: blackhole-dev (>= 0.2.1-1)

The reason  for this pull request is a bug fix in blackhole:
https://github.com/3Hren/blackhole/commit/e1bcb0d9868f083e396c835cbe57a3fb53f1c7b0

Blackhole is a header only library, so one has to rebuild projects which use blackhole with the latest version