You can remove Patina with sand paper from 1200 grit and higher wet/dry. You can also use a strop to polish it off, some car wax/compound and a towel works fine too.
A patina will change on you, lighten in some areas, darken in others, you will see blues, golds, purples rise and fall. It is all part of the experience of the knife.
That being said, if you want a dark solid grey, use a hot vinegar soak. Heat the vinegar, then lowe the blade into it, holding it up with a hanger through your lanyard/lanyard hole. And let it sit for 10-15 mins. Pull it out, rinse it off, reheat the same vinegar, then do it again. After your fourth soak, you should have a nice dark grey solid color.
Please don't soak the pivot area or you will cause corrosion on the pivot and cause a gritty feel to the action of the knife.
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Like Jr88 said, do a search. This topic has been gone over Plenty of times. You will glean Much more knowledge from reading the veritable encyclopedia dedicated to patinas than you will get in this one thread.
