Dear God, I hope so.
Although, I always assume if you see one insect inside the house, the odds are high there are many more of the same lurking about.
Black widows like to nest around wood, if possible. She has a web placed in a spot where she can hide inside the crevice of a cabinet. It's been difficult to see her out long enough to smack her.
A friend has one of these:
http://bugasalt.com/ and we're going to lie in wait and be ready for an assault. Hopefully, it won't take days for Ms. Widow to peep out.