Here's the deal ... Blanket -- MJ's youngest son, whose real name is Bigi Jackson -- filed documents this week, obtained by TMZ,  asking the court not to allow his grandmother Katherine to use Michael's estate money to fund an appeal of a previous ruling.

Basically, Blanket and his grandmother were recently working together to stop the executors of Jackson's estate from going through with a huge business transaction that they were very much so against.

The docs don't say exactly what the transaction was -- it's been shrouded in mystery throughout this saga -- but, based on recent reporting, it seems like it may be related to the deal the estate made with Sony to sell about half of Michael's music catalog for $600M.

Blanket explains he and his grandmother presented their arguments opposing the deal ... but ultimately the court ruled against them -- seemingly bringing the legal issue to a close.

However, Katherine's decided to appeal the ruling ... a move Blanket says he doesn’t want any part of -- and, more importantly ... he doesn't want his father's estate paying for her legal bills. As you know, he's an heir to the estate ... so that cash is pretty important to him.

The way Blanket explains it in the docs ... the appeal has little chance of winning, and he says he doesn't think it truly benefits the beneficiaries of the trust to continue the fight -- so, he simply doesn't want the estate to foot the bill for it ... something Katherine wants.

Blanket's also asked the court to use its best judgment to grant Katherine reasonable attorney's fees incurred from the pre-appeal battle -- which he agrees she's entitled to ... at least to a certain extent.

It's unsurprising Blanket's filing against Katherine ... just yesterday, executors of Michael's estate reportedly claimed that she receives a seven-figure allowance for the year -- money Blanket presumably would rather she use for the appeal instead of the estate's dough.