Black widows, being Theridiids, actually make great indoor spiders. They love to make webs in corners under furninture and such.

And the great thing about them is: they stay put. Once a Theridiid (spider of that famiily) establishes a web, they pretty much only move when they are distrubed or don't find enough food. So if you know that you have one and occationally feed it, you can almost guarantee it will stay there.

Except when it's a male, those will abandon the web once they reach maturity and start wandering around looking for love. But the great thing there is: Males are not medically significant! Only the adult females are and those will stay in their web until they die under the previous mentioned conditions.

So unless you have pets or kids that might unknowingly distrub the spider, you could reasonably keep those indoors.

Recluses on the other hand are active hunters, they will wander around. It's not like they want to be indoors, they just have a much higher chance of ending up there. But those should be put outside.