Venus Flytrap is turning black?

i have had my venus flytrap for a bit less than a month. I give it distilled water, an occasional fly, and it sits in the window of my kitchen with a good amount of light comes in. Lately, the little ones lying in the dirt shriveled up an turned black. i didnt think much of it until one of my tall big one started turning black. within 12 hours it was black! what do i do?????????????????