Fiends of Darkness ARE a blood summon. Blood (power) 2, Blood Magic (research) 2 IIRC. Mictlan just doesn't need the research. 5 slaves for 1 fiend.
Do you find Mictlan too dependent on the Fountain's blood hunt bonus? Has anyone had any luck with other gods for them?
What about battlefield magic? Those cheap priests could all cast Sabbath Slave while one of the more powerful priestmages casts Sabbath Master... but both Sabbath Master *and* Sabbath Slave cost a blood slave. That adds up fast compared to free communion. Without sabbaths, though, the priestmages seem somewhat lacking in magic options (we all know how good blood magic is on the battlefield

). It doesn't help that they're very expensive, and heat scale makes it difficult to benefit from Frost Fiends and even Ice Devils.
If Mictlan does need help, I'd do one of two things: remove the blood slave cost from Sabbath Slave (even if it is kept for Sabbath Master, you don't have to cast nearly as many Sabbath Masters) or give their priests a blood hunt bonus like Fountain of Blood (only not as big; they could hunt at 1 above their actual level). A national demon (like Harlequin) might help too, especially if it was sacred, but that would be more work.