Black widows like to school just like all the other tetras and prefer a school of atleast 6 but can live 3-4 per aquarium they grow to 2.5 inches so I think the 10 gallon would be too small, however im still learning how to stock a tank myself so im sure someone can correct me if i'm wrong :)
If you're having issues with stocking your aquarium, I use http://aqadvisor.com/ it's a fairly conservative calculator that takes fish behavior as well as size and waste production into account. If the tank is smaller than ten gallons, definitely not. They're a schooling fish that will be stressed out and pick on your betta if they aren't kept in a proper school.
They're great little fish and hilarious to watch! They spend most of their time zooming around the middle of the tank - not really big bottom-dwellers. I have a shoal of 5 and I think I paid ~$2.99 each.