What Causes Black Patches On The Faces While Taking Derma Roller Treatment?

I was getting derma roller treatments done at a skin clinic. The first 4 sessions were done very well and gave me good results. The maintenance session (5th one) was done by another doctor and he messed up the whole thing. It gave me bruises on my cheeks and now I have black patches on my skin- the way wounds leave patches after they heal up. The doctor is now suggesting cosmo peel to remove that skin so that my face can look normal in minimum time possible. Should I go ahead with this? I have gotten cosmo peels done earlier from the same doctor and it did work. But this time I am scared after one bad experience. Please suggest.