15 times more poisonous than a rattlesnake, maybe yes. But the amount they can inject would be so small to only cause pain and maybe cramping in a healthy adult.
Is the bearded dragon enclosure in the house? I have never found black widows in the house. They usually like darker areas. I would find them a lot on machines that hadnt been used in a while kept in sheds or shops. I would know when I was dealing with a potential black widow by the way the webs...