Does a rock face concrete block house need insulation?
In 1926 they didn’t worry about insulation, oil was cheap. The blocks have “open cells” in them that could be filled with insulation, but I doubt it. Usually reinforcing is installed in most of those cells. 

However, insulating the walls should be done after you 1) caulk all the cracks around windows and doors to cut down on drafts, 2) insulate the attic, heat rises so it’s VERY cost effective to add insulation in the attic, 3) up grade furnace, thermostat and air filter, if any, and upgrade hot water heater. 

Replacing single pane windows with thermal pane windows and insulating walls is usually not cost effective. 

I agree with @ecnerwal, get a copy of the heating bills.