I found this at The National Center for Home Food Preservation:

Why do the undersides of metal lids sometimes discolor?
Natural compounds in some foods, particularly acids, corrode metal and make a dark deposit on the underside of jar lids. This deposit on lids of sealed, properly processed canned foods is harmless.

That is extremely discolored, but if the jar was properly processed and sealed, it should be fine.

I have canned beans in the past, but have been very unhappy with the results. Several jars lost their seals, others had that same suspicious deposit under the lids, and I lost most of the liquid, there was a lot of siphoning. I quit canning them and just froze the shelled beans in Ziplock bags.

I suppose I ought to try again....

Annie