Canning grapes (unknown type) I separated the skins from the pulp/seeds, threw them in my blender with a little sugar and they turned brown during processing. The pulp also turned brown while simmering. I added them together, added sugar, began cooking and it became a medium purple color however, just before I finished the cooking process...the jam again turned brown. I was using all steel/ glass equipment.
Would/could it be the type of grape I used? It appeared to be concord grape from an old wild vine but does not tasted like concord grapes.
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.