That happened to me all the time. Its just because you minimized blockland and came back but if you play blockland in a window that probably wont happen.
when you see this, its time to "OMG RUN"
Twice a day, exactly 15 minutes after booting, I lock up twice. I lock for three minutes, a 15 second intermission, then I lock for another three minutes. I'm fine after that.
Your lucky...BL just crashes when I minimize it. I think it's an issue with Vista.