Mary J. Blige and Toni Braxton Rocking Out to Bell Biv Devoe

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No one can resist getting down on the dance floor when Bell Biv Devoe starts performing "Poison" -- not even Mary J. Blige and Toni Braxton!

It all went down at a birthday party last night for Toni's brother-in-law, music producer Vincent Herbert. In addition to working with Mary J and Toni, Vincent has also worked with Lady Gaga ... who we're told was also in attendance. 

Those girls are poison.