can bloodcursed arrows still be used after Harkon is dead?
Solution 1:
Harkon, like you apparently, is just someone who wants to use the power of the bow. He does not give it power. The bow's abilities when shooting the sun with both types of special arrow work fine after Harkon is dead. Though the "Clear Skies" shout is often required to find the sun in Skyrim.
The only thing I'm not sure of is if the bow needs to have available "soul gem power" to activate those abilities. Personally, I just keep it charged. It's remarkably low capacity makes it a good use for your smaller soul gems.