Black Zetsu told Izuna half the story and concealed the rest. He told him how, after Izuna's death, Madara had reached an understanding with the Senju and founded Konoha, only for the Senju to turn against him, and how he had endured hardship after the battle at the Valley of the End.
What he concealed was that Madara had read the Uchiha Stone Tablet and was plotting to revive the Ten-Tails and cast Infinite Tsukuyomi.
As for Naruto, whose body Madara possessed, Black Zetsu only briefly mentioned his identity as a member of the Uzumaki Clan and the Nine-Tails Jinchuriki. He said nothing about why Madara had possessed him.
An old schemer like Black Zetsu had his own calculations.
All signs suggest that Madara already suspects me. The reason he hasn't openly turned against me is probably that he wants to learn more about me—and about the black cursed Seal I created.
Madara cares deeply about this younger brother of his. How Izuna will react once he learns of Madara's plan is still unknown. If I rashly tell him, I might incur Madara's wrath. Better to let Madara handle it himself. In my New: Save Mother Plan, Izuna is only a minor, dispensable player anyway...
After learning what his elder brother had endured, Izuna seethed with indignation.
"My brother's proud soul has been reduced to inhabiting some brat from another clan!! Damn the Senju Clan! This is all the fault of those treacherous bastards!!! And that Itachi, who joined forces with the Senju to slaughter our clan, is even more unforgivable. Anyone has the right to kill him!!!"
No sooner had Izuna spoken than Madara entered Sage Cursed Fox Mode with his Nine-Tails Golden Body!
Izuna exclaimed, "My brother's form has changed...!!"
What a fuss this country bumpkin makes.
Black