March, obviously the later you leave it in the season the quieter it is but you risk the quality of the white stuff not being so good, although it should be absolutely fine beginning - mid march
Correct me if im wrong, I have never been to Borovets that late in the season but I know this is the case for Bansko
We always stay away from any Bulgaria ski resort during half term, xmas, new year and local holidays and we have always enjoyed pretty much uncrowded slopes and chairlifts. It is always going to be busy but their is busy and absolutely manic!!
beginning of march the conditions are great in borovets and ive never seen any mad crowds, maybe at the gondola first thing in morning...dont know where everyone goes though cos its always seems to be quiet at the top