Rulings

The enchanted creature can’t be enchanted or equipped by multicolored Auras and Equipment, it can’t be blocked by multicolored creatures, it can’t be targeted by multicolored spells or abilities from multicolored sources, and all damage dealt to it by multicolored sources is prevented.
Hybrid spells and permanents in the Return to Ravnica block are multicolored, even if you cast them with one color of mana.