Nick, if one was very blue already then it will only go darker if both stuck in together. I did the same with a red set I had, one was faded, one was not and I tried to dye both but the darker one went even more dark and the lighter one took only some of the dye colour!
Best thing I found was to dye the lighter one 1st to the same shade, then wack both in. Done this with another set and worked out ok.
If you want to dye them black they will both definately go black that's for sure and the same shade
Matt