Blind Diode Jefferson was forced offline when Dr. Mobius abandoned The Think Tank and left for the Forbidden Zone. Once his personality backup is retrieved from house #108, he speaks with a slow drawl, similar in style to a stereotypical Deep South Blues musician. When left alone, he may continue to ramble, spouting mostly-useless advice and anecdotes of by-gone times. When asked if he can play music, he says that he used to before Dr. Mobius took out his music drives - hence exacerbating his feeling of 'the blues'. Mobius used him to change the frequency of sonic emitters.
If the Courier asks Blind Diode Jefferson to modify a sonic emitter to the same function as is already in the Courier's inventory, it will be returned to you at near full repair.
As mentioned above, the slide show mentions that Jefferson saves The Big Empty from "sonic invasion" sometime in 2910 - whether or not this is a throw-away gag (let alone official canon), is yet to be seen, but if it is, this is by far the single furthest date ever posted in the Fallout universe.
When Old World Blues is completed, Blind Diode Jefferson says that there was only the lonesome road left to walk, which is a reference to the add-on Lonesome Road.
His animations (Music note animation and the bubbles) will stop when you talk to him.
This character is strikingly similar to the famous Texas Blues artist Blind Lemon Jefferson, with a diode simply being a component of a jukebox, just like a lemon (bitterness) could be seen to be a component of the blues.