Recently a few dark brown/black blotches under the skin of my penile shaft have appeared. i noticed it one day after masturbating. doesn't itch.

Here are some ...: What you see may have appeared lifelong and you just incidentally found them recently. Therefore, it's reasonable to just wait and see if there is no change in their size number, and shape. However, if changing and hurting, seek professional evaluation & counseling timely. Clinically, these macules may be harmless moles; so, just wait and see for now as suggested above. If needed, see Dr.