How do you safely get mildew off the back of a type cloth blind?

Try spot washing it with white vinegar

Try white vinegar (the label should say 5%)

As long as the blind comes in contact with the moisture, you‘ll have continual mildew, I’m afraid. I’m wondering if you mix up a weak solution of bleach and water then wet a sponge, almost wring out and then wipe the solution on the mildew if that MIGHT help. (Wear gloves.). This is more of a question than an answer because I’ve never had this problem. Then .... to, perhaps, protect the blind from further mildew, I’m wondering if spraying on a Scotchguard product MIGHT give some protection against the mildew. This answer is just off the top of my head. Perhaps someone else has had this actual problem and can be of better assistance. Good Luck!

Use vinegar. It will kill the mold spores, bleach will not.

Try to keep the window clean, use ammonia water to wash and vinegar water to rinse. The vinegar will kill any spores on the window to slow the process down of it happening again. Try the bleach water on an inconspicuous spot on the blind to make sure it doesn't change the color. Rinse the area with vinegar to do the same as the window. It will kill the left over spores that may still be in the fabric. Vinegar has excellent antibacterial and antifungal properties, it should keep it from happening as soon. Good luck!

I've used bleach to get rid of window mildew, but you really need to deal with the moisture. It isn't coming in from outside, it's condensation from inside. If it's just a problem during winter, I've known someone who taped plastic wrap across the window opening, in front of the glass. It has to be tight so it doesn't touch the glass. This keeps warm, moist air from touching the cold glass. If you have curtains that cover the tape, it actually works pretty well.

You’re welcome!