Dart frogs get their toxins from particular combinations of insects they eat. A lot of a particular ant and other insects that feed on potentially poisonous plants and fungi. When the frog feeds on these insects the poison is mixed up by the frog's body. PDFs are not Ceratophrys and with that said they do not create a powerful toxin like PDFs do. You cannot assume that just because this species can perform complex chemical concoctions that another completely different species can do the same. PDFs are small and cannot defend themselves from large predators so they produce a fatal toxin biochemicaly to make themselves inedible.
Ceratophrys are large frogs with a painful bite and the ability to fill themselves with air to make them appear even larger. Not to mention the deafening screams they can produce to safe off potential predators. You can't compare a PDF to a Ceratophrys. They're not even close to the same., BUT all frogs and toads secrete a kind of toxin onto their skin. Some are deadly, others just make them taste bad or irritate the eyes and skin.
It would be highly unlikely that a Black Widow would wonder into your frog's enclosure. Black Widows are very secretive and like dark cluttered areas with little activity. You're frog's enclosure would not be to her liking

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