Is the toe or the paw pad hot to the touch?
Good for you for being concerned and seeking out answers, of course without seeing the paws/nails I can only try to picture what you are describing. The dew claw( thumb) is usually larger than the other claws and can be a darker color as well, are you noticing a pus like discharge or redness around the base of the claw? If not than I don't think there is anything to be alarmed about.
A very dark claw color is what is called the Quick inside the nails...an it is normal that these contain blood.