opinel knives are a very good tool,the carbon blades are of a very good steel and keep an edge,as stated a wipe over with an oily rag is sufficient to keep it from rusting,the discolouration id perfectly normal,opinel do make a stainless steel version of many of their patterns,they too are very good but not as good as the carbon versions,
the best opinel i own is the 'opinel jardin' with its drop point its a winner.