Hi Jack.s
Late season in Borovets ( Bansko won't be much different) is March. Been there many times in march, is the best time/ most snow of the season is falling in the late season, but it can also get very warm - and melt very quickly. It's heavenly to sit in the sun/snow at the foot of the Markudzjik pistes, so take you chance or wait / check snow-forecast and book last - minute.