Well, he would not be able to defend, so his DB would be seriously hindered. I would say you get the same bonuses as if you were attacking from behind. (+35 if I am not mistaken.)
For me, I would say the non-darkvision individual would have maybe half their normal DB, and if they were going to go all-out defense (like weaving their sword around them and turning to cover as much area as possible (not a gret defense, but something), they would be able to take about 25% of their total OB and put it towards defense. Of course, the combatant that does have darkvision still gets the +35, mentioned above. So, while not completely defenseless, the effectively blind person is at a serious disadvantage.
Other senses could help them. Like if they had exceptional hearing (much better - and/or extensively trained - than a normal humans), they may be able to get a better idea at where their opponent is. This would give them a better ability to defend. There are many ways how to handle that situation, from making them do an Awareness Senses: Hearing check and using the results to modify how well they defend, to just saying they have superior hearing so they are able to defend at 50% (2 points of OB for 1 point of DB).