If it's the primary consistency, then there's a bacterial imbalance (mild to more serious - usually mild). If it's only occassionally, that's a "cecal poop". If it's almost black, that usually means it was the color of dark chocolate pudding before it dried. (Those droppings dry darker).
I wouldn't worry excessively. What I would do, instead, is give yourself a little insurance policy by boosting their essential beneficial gut bacteria right now with probiotics and possible PREbiotics.
Probiotics (living essential bacteria to replenish those lost in illness or stress): yogurt (plain white - don't use while medicating with --cycline or --mycin meds. Use about 1 teaspoon per 2 cups or so - just not excessive amounts. Think in terms of what a serving for a human would be if we were their height.) Acidophilus capsules or tablets from the grocer/pharmacy/health food store. Probiotics for livestock, most commonly found in the horse aisle: probios, fastrack, etc.
Prebiotics (nutrients that nourish the bacteria so that they thrive): apple cider vinegar (also a probiotic - use organic for bacteria), applesauce (preferably non-sweetened in small amounts).
Hope this helps.
