Praise be to Allah.
The end of menses is known from one of two signs:
The first sign is emission of the white discharge, which is a white liquid that is known to women.
The second sign is complete dryness, meaning that if the woman puts a piece of cotton and the like into that place, it will come out clean with no trace of blood or yellowish discharge on it.
The blind woman can be helped by her mother or sister, who can look at the cotton or cloth, and if she sees the white discharge or sees that it is clean, she can tell the blind woman that she is pure [i.e., her period has ended].
Maalik narrated in al-Muwatta’ (130) from Umm ‘Alqamah that she said: The women used to send to ‘Aa’ishah, the Mother of the Believers, vessels containing pieces of cloth on which there were yellowish traces of menstrual blood, asking her about prayer, and she would say to them: Do not hasten until you see the white discharge – referring to the end of menses.
If this is what seeing women do, then it is more appropriate for a blind woman to do it.
And Allah knows best.