Not all poison darts are blue, quite a lot of them are black and yellow or orange, orange and blue or blue and black.
I doubt it's camouflage so much as a warning. The different colours may be suited to the colours most visible their main nemesis, as some animals have near or entirely monochromatic vision. Humans have pretty terrible vision in terms of the entire electromagnetic spectrum (it's a hair thin line somewhere in the middle).
The poison in those frogs is actually one of the most toxic chemicals known to man, it's about as lethal as VX nerve gas in terms of the dose. Maitotoxin, from a fish in Thailand, is about 20 times worse and Botulinum Toxin is the worst, at over 2000x stronger than the frog and nerve gas, which is why Liza Minelli's face looks the way it does now.
That aside, they do look incredibly cute and very colourful. They also wake up during the day and go to bed at night, unlike most other froggies.