Summoners have an outsider pet called an eidolon.
As the summoner levels, you “build” the eidolon with evolutions like extra claws, energy resistance, breath weapon, and so on.
At 10th level, a summoner gains an ability called “aspect,” which allows you to apply a weak evolution to yourself instead of your eidolon.
Outsiders by default have darkvision.
Therefore there is no evolution to give your eidolon darkvision–because they already have it, there’s no need for it.
But that means a you couldn’t use aspect to grant yourself darkvision in the way you might grant himself claws or energy resistance.
In Ultimate Magic, I added a low-light vision evolution, just in case you wanted to add it to your eidolon (and therefore could use aspect to add it to yourself, if you so desired)
But there really should be a darkvision evolution just so you can add it to yourself with aspect. And, so it’s not useless except for that niche application, it should be useful to the eidolon in some way. Therefore, I suggest this:
Darkvision (Ex): An eidolon gains darkvision with a range of 60 feet. If it already has darkvision 60 feet, it instead gains darkvision 120 feet.
(A third-party publisher might have already tackled this, but if so I haven’t read it.)
(It has to be 2 points because it’s better than low-light vision, which costs 1. Sadly, it’s not really as good as the other 2-point evos, but we don’t have the option for half-points.)