Depending on the circumstances of the assessment, some allowance might be required in marking the written work of blind and visually impaired students. Particularly in handwritten work, there may be phonetic mis-spellings and confusion in the spelling of same sounding words that should not be penalised.
Blind and visually impaired students may not be able to access some internet sites and so should not be penalised for using fewer primary or secondary sources. In a given amount of time they are unlikely to be able to read as widely as their sighted peers. Visually impaired students can also have some difficulty in using referencing systems.
A blind or visually impaired student is not be able to access handwritten feedback. Instead you could send it using e-mail, via audiotape, or in discussion with the student. When documents are submitted for assessment electronically, tutors can add comments directly into the document.