The rapper made the allegation online Sunday amid his ongoing feud with Chrisean ... with the latest hurdle between them now being about their child together -- which Blue just announced the other day was not, in fact, his after he allegedly got confirmation.

On the heels of that drama, Blue tweeted out ... "Being tatted ona hoe is not a flex you literally f****d cardi B husband couple weeks ago I’m tired of n****s looking at me while they f*****g you get the rest of em gone asap please."

It looks like Chrisean is tweeting back at Blueface right now and saying it ain't true, to which Blue responded with specific dates and times ... "So you ain’t f*** cardi husband November 10th at 4am at their house in LA…I’m making this up ?"

Welp, Offset chimed in on the back and forth ... and he's saying yeah, Blue is making it up.

Offset wrote in direct response to one of Blueface's tweets, "I ain't never  talk or touch that lady. Real talk man you need some help!" To that, Blueface appears to have replied and said this ... "If the truth tears you down you living a shameful life and I feel bad for you cuz I ain’t never gone lie on my d*** if I hit it ima admit it on SY."

As for Chrisean -- who's been feuding a lot with Blueface lately, especially pertaining to her baby boy -- she threw out her own accusations at him ... including claims that she'd seen Blueface's Internet search history of late and saw some gay porn results, which he denied. She, of course, also denied his Offset claim, saying ... "U just crashing out making up s***."

Now what's interesting about Blueface's accusation here is the fact that Offset and Cardi do, in fact, appear to be going through more difficulties -- as she's been cryptically implying she's over him ... seemingly referring to him as "dead weight" and straight up unfollowing him.

She hasn't clarified what exactly the problem is ... but considering his history of alleged infidelity, something tells us it might be about that -- especially with this latest development.