No, it is not necessary to chum for blacks. Just makes sure that you are anchored over structure and use skimmer clam in the spring and green crabs in the fall. One good tip is to use two anchors as it will hold you over the structure better.
Absolutly chum for them but not like you are thinking. When using green crabs, scatter the legs and shells around the boat. Do the same with clam shells or mussles. The sound of crunching leggs and shells will draw more in under your boat. If your fishing in a area with strong tide, this won't work. I would even crush 2 or three green crabs and toss a few live ones into a chum pot. Those live ones dancing around in the pot alsong with the smell of the dead ones really brings in good blacks. Don't feed them, Just get them into the area.
I heard of guys on the south shore tossing a few fiddler crabs in a clear glass bottle and sending it down on some mono. That also drove them crazy.