Yes, Hobie produced black non-EPO rudders for a period during the '90s. The material was called PCG. They were solid molded plastic rudders that were not as light or stiff as EPOs and the foil was not as precise.
There are a couple ways to tell what rudder you have. First would be weight. EPOs are significantly lighter than solid plastic. You will be able to feel the difference just by picking one up. Second would be to look in the rudder bolt holes. If you look inside and see foam core, then you know they're EPOs (however if someone drilled the holes oversize, filled with resin, and then drilled you would not see the core). Third, there is a seam around the whole mid-line of the rudder. It is most noticable along the top edge of the rudder where the two halves of the rudder are joined. It is ground off around the rest of the rudder so may not be visible. Fourth would be if there is weave/fibers showing through on the surface. However this only occurs on rudders which have set out in the sun for a long time and have been UV damaged. If the rudder was well cared for, there will be no visible weave.