I assume that you are indeed talking about blur occurring in the negatives rather than in the print (this could happen as a consequence of negative popping). Considering your statement that the negatives were very dense, I think the blur might be due to halation. As you state that you didn't modify the development, the whole film is probably badly overexposed.(posted 8982 days ago)Although this is only conjecture, it sounds plausible that this might be a consequence of a slow diaphragma. If the diaphragma has not closed to the intended aperture (as a consequence of, e.g. dust or an aged lubricant), the shutter opens while the aperture is still too big. This might lead to overexposure.
This would be ruled out, if more than one lens was used for the negatives in question and they are all equally overexposed. (It is not very likely that all lenses fail at the same time.) If you used only one lens, detach the lens, actuate the diaphragma lever on the lens manually, and watch the diaphragma. If it closes in slow motion, it must be cleaned.