I flashed marshmalow (android v6.0 ) on my blackview P2 , rooted it with the root method provided in development options (actually superSU is the root binary), installed flashify and gave it root , then used flashify to install the twrp 3.0.3 img file.

Everything worked well so far

I then unrooted the phone successfully through the development options and the superSU app.
But when I tried to flash magisk through twrp , the flash aborted because the boot.img was patched.

I don't understand how the boot.img can still be patched when I uninstalled superSU per-instruction.

How can I unroot superSU and make sure it replaces the patched binaries with the original boot images ?