Aunt Blemish is a relative of The Addams Family.
Once, while collecting documents to trace the Addams ancestry, Morticia saw an old family photograph of what she thinks is the old Addams barn. Gomez says that it's really a picture of Aunt Blemish, but when Morticia asks him if he's sure, he admits she may be right.
The Addams Family original series episode:
The New Addams Family episodes: