These black spots are corroded areas on the stainless steel. This is due to contact with either blood, acids etc. The difference between stainless and normal carbon steel is the addition of chromium. This element aids in the resistance to corrosion. Stainless in the right conditions will corrode, even marine grade stainless. It is our opinion that the steel has been exposed to conditions where this has allowed the stainless to blemish. The metalwork could be bead blasted to remove this, otherwise only regular lubrication can prevent this from happening again.