There is debate as to why Singer made some machines blackside - most think it has something to do with availability during the war years. They are no different except the finish, and less common for the most part.
I have a 66 and a 128 that are blackside - and I've put together one set of as many blackside attachments as I've found so far. Around the mid 1940's, you'll find a lot of sets of attachments that have a mix of blackside and nickel finish attachments, and often it's specific parts of attachments, and not the whole. Speculation is that Singer made the parts in bulk, so what got made in bulk at that time was blackside, and some parts never got made that way.
Here are some pictures to show what you may find out there.
This picture shows the adjustable hemmer, part blackside and part nickel. It's also not uncommon to find a machine that has a mix, like the plates will be blackside, but the needlescrew and bobbin winder guide will be nickel. Most often they were on "godzilla" finish machines. BTW - for what ever reason, Singer apparently didn't make belt guards in godzilla finish, so the godzilla finish machines will have a japanned finish belt guard - and they came that way from the factory.
