Let's assume a person can no longer see, but he can play a musical instrument. He wants to read notes and learn new pieces. Is there any kind of notation system for such blind people?
There is a braille system for music that uses the same 2X3 configuration of dots as literary braille, but ascribes different meaning to the patterns. The Wikipedia page about musical braille lists the most common symbols.
The National Federation of the Blind maintains a page dedicated to braille music resources.
Here is a link to a non-profit project to provide braille sheet music for visually impaired musicians. Braille sheet music can be downloaded for free here, or printed copies can be requested for those who don't have access to a braille printer.
Yes. Blind musicians can use Braille to learn new pieces, but there is only a limited amount of music available in this form. Learning by ear is an important method too of course.