O-kay another theory-.
1- Dadi was black mamba in the past. Or dadaji was black mamba, and dadi took over his name after he died, and continued his work with this name.
2- Dadi could be Black mamba, but she didn't hire this sara to kill Vansh, and sara is lying. She could have been hired by someone else to kill vansh (my guess is vyom), but she took black mamba's name to confuse Riddhu and put the blame on dadi. Sara could be vyom's ally. If there's someone who knows dadi's past it's vyom. So he could also be knowing about her being black mamba🙄🙄