What are the chances a black widow is found in a house?

Whatever it was, it probably wasn't a widow. Even when they're upset and frightened, widows don't move too fast; they're just not built for running. Widows also don't tend to stroll around too much; they like to make a web in a likely spot and just hang out there, catching and eating food and growing as fast as they can. There are other (harmless) black spiders that wander, and yours was very likely one of those. Perhaps it was Boris the Spider reborn.