As the title asks, can I cast Aphotic Wisps on a creature like Dross Prowler that already has fear and is already black?
In the same vein (I don't see why it would be different, but Magic is complex), can I cast 2 copies of Aphotic Wisps on the same creature and will they both resolve?
You can target any* creature, it doesn't matter if the creature is black and/or has fear. This is because all the card says is "Target creature", not something like "Target non-black creature without fear". Dross Prowler is a creature thus it is a valid target for "Target creature".
114.1a An instant or sorcery spell is targeted if its spell ability identifies something it will affect by using the phrase “target [something],” where the “something” is a phrase that describes an object, player, or zone.
*barring things like hexproof, shroud and protection
Sure, there's no reason you can't do that. All Aphotic Wisps requires is a "target creature", it doesn't care what color it is, or what abilities it has. You could cast as many Aphotic Wisps as you want on the same creature and they'll all resolve, but only the first one to do so will change the properties of the target creature. You will get to draw a card for each Aphotic Wisps that resolves, though.
