When Black Zetsu first said that simply stripping away that chunk of flesh would remove the Flying Thunder God Mark, Obito had been sorely tempted.
After all, he had powerful regeneration. With hashirama cells in him, the worst that could happen was losing another chunk of flesh. Only half his original body remained anyway; replacing a little more hardly mattered.
But Kakashi had put that Flying Thunder God Mark, which looked just like Kamui, in such a fucking stupid spot-right on his lower abdomen. How was he supposed to cut that out?
Surely he couldn't just leave a scar the size of a bowl.
So that option was ruled out outright.
And when he heard about what Madara had left behind, Obito thought of Black Zetsu's identity. Supposedly, he was a creation of Madara Uchiha's Yin-Yang Release when he awakened the Rinnegan, an incarnation of Madara's will.
He was also something Madara used to keep an eye on him, so Obito maintained a certain wariness toward Black Zetsu.
Madara treated him as a pawn, while he treated Nagato as a pawn. His relationship with Black Zetsu was somewhat like his relationship with Nagato, yet different.
As for what Madara could have left behind that could erase a Flying Thunder God Mark, a mark left by Space-Time Ninjutsu, Obito could think of only one answer: Yin-Yang Release.
Yin-Yang Release could be lofty, but it could also be quite basic. At its simplest, it split into Yin Release and Yang Release. Ordinary Genjutsu and Medical Ninjutsu were applications of Yin Release and Yang Release respectively. Slightly more advanced examples included the Akimichi Clan's Multi-Size Technique, the Nara Clan's Shadow Control Technique, and the Yamanaka Clan's Mind Body Switch Technique.
These were common throughout the Shinobi World, but combining Yin Release and Yang