I don't know about that. Concord's taste and aroma are pronounced. It may overpower any Vinifera you add, they are more subtle. I have used northern grapes to perk up a bland California white and a little goes a long way, 4% is pushing it at times. That was with Diamond, which does not have as pronounced a taste as Concord.
You may want to consider staying in the family of the type of grapes though. Welches Niagara is usually well thought of and may blend well with Concord. You won't ruin it by blending, you just may have spent more money on the Cab to have it all taste like Concord. You might like the Cab as is and decide not to blend it too.
Blending in an art, not a science. You won't go wrong making other wines to blend, but if you have a wine that came out well there is no good reason to mess with success as I see it. I have made literally hundreds of small batches which all turned out well, but of those maybe
5 were perfect as I see it, they could not have been better as far as I was concerned. I am a nitpicker though...
Joe