You are right, but there are some hints that there were, aside of light worshippers, some Arcane Powered Holy Priests.
There were also some "arcane priests" in Suramar, who used moon magic to deal damage and heal themselves, so we can assume that arcane can provide ways to look like a priest. If nightborne can, they may be some among blood elves. However, we don't know if playable Nightborne are arcane priests or rather an oversight.Blood elves follow, or at least did when they were high elves, the Holy Light just like humans and dwarves. Unlike the two others however, who derive their power from faith alone, high elf and blood elf priests seem to derive theirs from other sources. The first of these was the Sunwell, brought to end by the Scourge during the Third War when it was used to resurrect the Kel'thuzad.