Legally, the person who created a work own's its IP even if they didn't trademark the work. As such, only Eul and Guinsoo (who, hilariously enough, works at Riot Games (League of Legends) ) can claim the copyright to the work.
If they don't contest valve, I don't see why Blizz can. Unless there is a clause in the map editor which states they own everything anyone makes in it.
IceFrog works at Valve now (on DOTA2) but he started modding it in 2005; 2 years after Eul's original inception of the game, and his work was just a refinement of the large strides Guinsoo made in turning DOTA into what we know today.