Is a male Black Widow poisonous?

Yes, the male Black Widow has venom. But they are very shy, much smaller than the female, and much less aggressive. Their diminutive physique means they have small fangs, small amounts of venom, and they can't deliver the kind of deep bite (and high venom volume) that the female can. Unless you saw a male Black Widow in the web of a female courting it, you'd likely never notice or otherwise encounter one. A link can be found below so you can at least see a picture of a "couple" of these arachnids. You're looking "up" at them from their ventral side, and the red hourglass of the female is very apparent.