With adult frogs, the males sometimes appear thinner than the females-- mostly somewhat more slender in the abdomen. But if both are eating well, and one is substantially thinner in the thighs and back, there's probably another problem, perhaps a parasite overload that pflame mentioned. (You did not say which blue darts these are--D. azureus or the blue morphs of D. auratus?) This might be a factor re: competition and intimidation, especially if he has two adult female azureus.
Finding a herp vet is often more easily recommended than done. You can check out the following link, go to Members, then find your area, which should list members of the ARAV.
Association of Reptile and Amphibian Veterinarians
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
7 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
6 P. terribilis mint and organe
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus
2 P. lugubris