Hi i have a question about a the rules around what forgotten plans plot card actually effects and if it effects the mutal enemies plot card since its a when revealed plot card.

forgotten plans: Treat all plots with a name other than Forgotten Plans as if their text box were blank during every phase except the plot phase.

mutal enemies:When revealed, if you have more than one opponent, choose an opponent. You and that opponent cannot declare Military challenges against one another this round.

now since mutal enemies is done in plot phase does it still count in all the other phases with this card out ?

Yes. This is because the plot sets up a lasting effect during the plot phase. So even if the plot is blanked, there is still a lasting effect that says "You and Player B can't do power challenges against each other this round."

"So even if the plot is blanked, there is still a lasting effect that says "You and Player B can't do power challenges against each other this round." "

so that means the lasting effect from when revealed plots are un-effected by the blanking effect of forgotten plans?

That is correct.