The green-and-black poison dart frog (Dendrobates auratus), unlike the strawberry dart frog, is actually quite poisonous. Its poison is strong enough to kill a human but that's no reason to fear it. See, they will only release their poison if they feel threatened. So as long as you treat it with respect and stay calm around them they likely won't release any poison (though I wouldn't recommend trying to handle one).
This was another of the frogs in Costa Rica that I really wanted to see and finally did near the end of my trip while at the Pacuare River Eco-lodge. It was very shy and once found didn't stick around very long. One more Costa Rican dart frog to go. Any guesses as to what it is?