Is it actually blocked or is it clean but she can't go? If it is blocked, use a warm water and towel to gently clean it off. If she is constipated I have read that a drop of molasses in a tablespoon of warm water can be given to her by a dropper. Like 6-12 sips a day, making it up fresh so it doesn't ferment. Make sure you don't drip it in her nostrils.
If its pasty butt you can use a warm wet washcloth or papertowel and gently hold it against her vent to soften up the poop. Gently wipe the poop off but dont pull it or you might hurt it. If that doesnt work, you could try soaking its little butt in a small dish of warm water then wiping it. I hope this helps.