I"m forging a blade with high carbon (pure carbon) core and wrough iron clad. Because of the WI, I have to work at very high temperature, which is bad for HC. As far as I know, it creates large grains.

Is there a way to reduce the grain size?

Yes, a proper normalizing cycle can accomplish this.  My recommendations are as follows:

1. Heat stock to well into the austentitic range and hold there for a couple of minutes then quench in oil to regularize grain (make even large grains rather than large and small)
2. Heat stock just barely into the austentitic range (decalescence) the air cool till stock goes back to magnetic to reduce grain size
3. Heat stock to below the austentic range (just barely non-magnetic) and let air cool to relieve stress then let cool till goes fully black
4. Heat stock to where it is just barely showing a hint of red in a darkened room (subcritical anneal) to further relieve stress and let air cool to handle able temperatures and grind to shape prior to final heat treatment cycles

Good luck