cbwjm
Seb-wejem
Nothing in the rules prevents it. Fluff doesn't really prohibit it either. Some would probably encourage it i.e. a cleric of asmodeus/fiend warlock or cleric of an elven deity/fey warlock. A lot of clerics I can see secretly delving into forbidden arts without the knowledge of the church. Also, there isn't really a penalty (at least not a mechanical one) for a cleric turning away from their god, not like in earlier editions where they lost their cleric abilities. Now a cleric can get invested with power and then turn around and flip their god off, keeping the divine gifts they have been given.