Impossible. A black widow and a tarantula are different species, therefore they would be unable to reproduce. There's no need to worry about hybrid spiders spawning anytime soon unless some crazy lab experiment escapes.
Why would you want to do that?! Create a deadly creature. But unfortunately for you, highly unlikely. They don't even belong to the same genus. (Note that they should be the same species to breed).
I went on a site on this topic but can't remember the name, If I'll remember I will let you know.
I would say no. They are not compatible, it'd be like making a goldfish shark. They're both fish, but different it still would not work.
It would be as deadly as the black widow and look like a widow,but the size of a tarantula
