Do potassium channel blockers affect the resting membrane potential?
Potassium channels participate in controlling the concentration gradient of the cell.
  


I would rephrase this to "Sodium/potassium pumps establish the concentration gradients of sodium and potassium across the cell membrane." Potassium channels do not.


  I have found evidence in articles supporting the first claim, but I
  don't know much about the second.


Right; because the channels don't set up concentration gradients.


  Do potassium channel blockers affect the resting membrane potential? 


If they block the resting state "leak" potassium channels, you bet. The resting membrane potential is largely set by the electrical conductance through those channels. Blocking them would greatly depolarize the cell compared to typical resting membrane potential values. But there are many different potassium channels. You may be thinking of blocking the delayed rectifier potassium channel, which shouldn't do much at all to change the resting membrane potential.


  Would they significantly alter the concentration gradient? 


Probably not, since though the cell would be losing less potassium through this conductance, what matters more to the concentration gradient of potassium are the sodium/potassium pumps.