Polish the blade with something like Simichrome, Flitz, etc. The 'patina' is just black iron oxide, which is what these polishing pastes are designed to remove. Make sure to use a polish that's made for hardened steel. Some polishes are made for softer metals, like gold & silver, and won't be as effective. I wouldn't use sandpaper for this, without first trying the polish, unless you're not worried about the scratches.
As noted earlier, personally, I wouldn't mess with that natural patina. It's a good thing, which helps protect the steel from red rust. If you just want to clean the blade (tree sap, dirt, glue, etc.), I'd just scrub with #0000 steel wool and WD-40.