im making a rogue for the first time and was wondering if it should be or undead or blood elf and what spec im pretty sure i want to be assassination with poisons for dungeons but idk whats the best combo between race and spec

Race honestly does not matter unless you want some minor boosts like dwarf if you want to wield two handed maces or night elf if you want the stealth boosts. But between orc and undead, Choose what you think would look cooler and be more fun to play, and choose any Spec you would like to play, it doesn't matter, have fun.