Why does mulberry have so many different endings? (桑葚、桑椹、桑甚、桑黮)？
桑葚（sāng shèn），又作桑椹，桑树的成熟果实，为桑科植物桑树的果穗。又名桑椹子、桑蔗、桑枣、桑果、桑泡儿，乌椹等。
黮
dàn
云黑色：“当时黮闇犹承误，末俗纷纭更乱真。”
shèn
古通“葚”，桑果：“食我桑黮。”

So it seems that 桑椹 and 桑葚 are the same. And 黮 is the interchangeability character of 葚 in ancient time. While 甚 should be a wrong word since I didn't find any explanation related to 桑甚.