Blocks do have bearings, but I do not believe there is any real maintenance that can be performed on them unless you have the proper tools to disassemble them. Are you referring to your mainsheet blocks or jib blocks? Are your blocks the stock Seaway blocks, a low profile or higher purchase Harken setup, or some other brand?
I've run into the same problem. My mainsheet blocks are a low profile setup by a brand I had not heard of and cannot find, that I believe has gone out of business. They do not spin as freely as they should, and I have found no way to service them, so I'm looking at replacing them.
My jib blocks, on the other hand, are IFL (Oxen/Oschen) self ratcheting blocks, really quite handy, though very few 18's that I've seen use them. Murray's will service these blocks if they need it. (http://www.oxenblocks.com/oxen.pdf