I made a similar mistake. I checked the label of a gallon jug and did not see any perservatives listed. But most of the jugs in this cooler were smooshed probably from being in a bottom box in an overloaded truck. So I saw a cooler opposite from the one I pulled this jug from. They had the same labels so I did not look for preservatives, instead I was looking for non smooshed jugs. I bought 4 gallons total. I made a 5 gallon batch of Brandon O's Graff pitched the yeast. In the morning I got up and there was no airlock activity. So I got parinoid and went to check the recycle bin. On 3 of the jugs I bought there was a small label on the back saying it had k-sorbate and sodium benzoate!!!! They had the same exact label as the jug I originally picked up so I can only assume that it too had preservatives in it.
So I went to work feeling all bummed and was thinking about stopping by the LHBS to get more ale yeast on the way home. I ended up not doing that and just giving it a chance to get going. I figured the 1 gallon of malt might dilute it enough to give it a chance to get going. I got home to find the airlock pushed up and it bubbling every 2 min and 30 seconds. I make mead also so I had some nutrients and energizer handy. I dumped some of that in the jug and by the next day it was bubbling every 40 seconds or so. 1 month later it is still going and I get a bubble every 14 seconds. In the mean time I started and am about ready to bottle another batch of Graff.
Moral of the story. Relax, dump some yeast nutrients/energizer in if you have some, and hope for the best. Next time buy only cider from a mill and call them before hand to see if they use preservatives. Calling ahead of time has saved me 2 car rides so far! Good luck with your batch, I think because you blended it you may have a fighting chance. It may not turn out as good as it could have, but it might not be a total waste either.