I normally think of black boots on a prop as being for de-icing. However, the Extra 300 is not suitable for known-icing conditions, or even IMC flight. (It's AFM says "Only VFR flights at day are allowed").
Considering the Extra 300's aerobatic mission, the gyroscopic loads on the engine are fierce. The upshot is that the manufacturer would not include a single extra gram more than necessary here.
So why are they there/what do they do? It has the owner and I stumped.