Blackview p2 comes rootable through a signed internal superSU binary ( settings -> about phone -> 7 taps on "build number" to enter developer mode -> activate root in developer mode and enter the code 252258439 when prompted -> reboot)
I prefer magisk as a rooting binary because of the many advanatges it has over other traditional rooting methods (superSU , superuser kingo root , kig root etc)
If you root your blackview p2 through the internal superSU binary provided through their rooting method - and you unroot later (through the unrooting method provided and the superSU app) - YOUR boot.img REMAINS PATCHED.
Unrooting does not unpatch the BOOT.IMG file (I experimented with both a superSU unrooting binary zip file you flash through twrp , and through the superSU app) and so other root binaries like magisk cannot be flashed (they need the unpatched boot.img file).
So I did the next simple thing , I reflashed ONLY the boot.img file from the ROM through the sp flash tool and everything worked
The latest sp flah tool https://androidmtk.com/smart-phone-flash-tool
So , after unrooting superSU with which ever method you use , you have to reflash the boot.img file too .