Charlie: Thanks again! I know about the Chocolate Church and it is too "new" - has to be a place where folk and blues musicians performed in the 60s, specifically. There is a project called the Mississippi Blues Trail - you can find out more by Googling that name - and they are placing historic markers at sites where blues and folk/blues musicians performed.
Appreciate all the help, and if a suitable site for a historic marker is found in Maine I'll definitely post it here! Best,

Mary Katherine