Can remember many moons ago, late 50's early 60's (still in short trousers!) watching someone at Southend with a bucket of whitewash and a long handled brush tracking a helicopter (try as I might cannot remember what).
He simple loaded the brush with whitewash, stood underneath the whirling blades and poked the brush up till it touched a blade.
Then they stopped the engine, tweaked the (white) blade, wiped the blade clean and repeated the process.