Is it permissible to take off the khimar in front of a blind man?

There is nothing wrong with a woman taking off her niqab or
khimar in front of a blind man. 

It was narrated from Faatimah bint Qays (may Allaah be
pleased with her) that the Messenger of Allaah (peace and blessings of
Allaah be upon him) told her to observe her ‘iddah in the house of Umm
Shareek (that was when her husband died), then he said: “She is a woman whom
my companions visit. Observe your ‘iddah in the house of Ibn Umm Maktoom,
for he is a blind man and you can take off your garment.” Narrated by Muslim
(1480). In another report it says: “If you take off your khimar he will not
see you.” 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was
asked: What is the ruling on observing hijab in front of a blind teacher? 

He replied: 

There is no hijab in front of a blind teacher because the
Prophet (peace and blessings of Allaah be upon him) said to Faatimah
bint Qays: “Observe your ‘iddah in the house of Ibn Umm Maktoom, for he is a
blind man and you can take off your garment.” End quote.  

Fataawa Noor ‘ala al-Darb by
Shaykh Ibn ‘Uthaymeen (Fataawa al-Nisa’/ 170). 

Shaykh Saalih al-Fawzaan said in al-Muntaqa min Fataawa
al-Fawzaan (3/question no. 410): 

The correct view – and Allaah knows best – is that she does
not have to observe hijab in front of a blind man, i.e., covering her face
in his presence. End quote. 

And Allaah knows best.