In mtg, what happens if guardian of the guildpact blocks malakir bloodwitch?

I played against a vampire deck with a deck with guardian of the guildpact and tried to block malakir bloodwitch. Malakir bloodwitch has flying and protection from white, but guardian of the guildpact has protection from monocolored (guardian of the guildpact is white). I can't figure out what happens!! Help me!