Michael Blackson knows exactly what's really behind Beyoncé's recent lightened appearance, and apologies to Tina Knowles, but he says it's really about a bedroom "Renaissance" for Jay-Z!!!

Mike dropped his juicy theory on us when we got him at LAX, and asked what he makes of all the fan remarks about Bey's appearance during the L.A. premiere of her 'Renaissance' film.

Unlike Mama Knowles, who said the silvery/platinum look was simply about sticking to the theme of Bey's tour -- Michael thinks it's really about the Carters mixing things up in the bedroom. As he sees it, Beyoncé's just giving her man "another woman."

Jokes aside, MB did totally dismiss the haters who claimed B was bleaching her skin ... and he explained why he's filing that under utterly ridiculous.

As we reported, Tina also went to bat for her daughter ... lashing out at anyone spreading the notion Beyonce is anything other than a proud Black woman.

We're not sure Tina would be any more down with Michael's "theory," though.