Now this is what I call keeping a common name simple and, yes, there is a second Brachydiplax species which is tagged a Darkmouth (though I haven’t seen it). B. denticauda is native to Australia, Indonesia, PNG and the Solomon Islands.
I had just a single encounter with this species at the the St. Lawrence Wetlands in Queensland, 2019. There is little said about it so mostly what I know is that it seems typical of other Brachydiplax species in the male being powder blue. The palemouth name looks appropriate, though.