Canning grapes (unknown type)
A lot of jam recipes call for adding some lemon juice to help guard against oxidation, which could be your source of browning.  Did you use any kind of acidic additive?

Turning brown, in and of itself, is more a cosmetic/aesthetic thing.  If it tastes fine, I wouldn't worry about it.  Label it, so you don't get it mixed up with other jams/preserves that are normally brown.