And harry used it to raise a zombie dinosaur. Which I been under the impression had still stained him with detectable black magic, same as his earlier run-in's with Black Magic had stained him in a detectable way.
And stabbing a Senior Council Member in the face is punishable by death but is not Black Magic Council "Justice" is a wildly different topic. 
Here's a nice metaphor... Imagine a clear glass of water as someone's source of magic, this magic is effected directly by what they use it for, what they actually manifest, ect. squirting food coloring into it, say red for using fire magic, black ink for black magic, ect. well, once you squirt some ink into it the resulting alloy/mixture isn't ever gonna look quite the same even if you start dumping white out in to try an balance it... imagine the best you could hope for after a time is a deep purple.
On the hat thing it's long been my belief that
Harry will wear a 'black hat' instead of wielding it in staff form, to make a joke on his own term for evil magic users, the blackhats... Now wither MM Harry in fact is already wearing the hat, and if after MM Harry always remembers wearing the hat, is the interesting thing to me
