What are the black boots on this Extra 300's prop?
The inner portion of the blade is protected by a self-adhesive PU-strip, unless the blade is equipped with a de-ice boot.

The outboard edge as you can see in the very lower-right corner is protected with a stainless steel erosion sheath. Since the inboard section is slower (radial speed), its protection is a lighter polyurethane-strip/tape.



The Extra 300's service manual lists the 3-blade prop as one of two MTV-9 series (you can see the MT logo already in your photo). The above quote and illustration are from the MTV-9 series operation manual from mt-propeller.com. An example tape is the 3M 8663HS, which comes in black or transparent.