Distiller: Convert RGB EPS to CMYK PDF with rich black

I send RGB EPSes to my printer who outputs them on a digital press (a Konica-Minolta C8000). I know, CMYK would be better but RGB has been working fine up until recently.

Now, solid black backgrounds print washed-out. I'm still trying to pin down what their workflow is, but I'm assuming Acrobat is involved. So I created a very simple RGB EPS:

%!PS-Adobe-2.0 EPSF-3.0

%%Title: black box

%%Pages: 1

%%BoundingBox: 0 0 100 100

%%EndComments

newpath

0 0 moveto

0 100 lineto

100 100 lineto

100 0 lineto

closepath 0 0 0 setrgbcolor fill

showpage

%%EOF

And, sure enough, no matter how I change Distiller's color settings, I can't get the resulting CMYK PDF to have a rich black -- it's always 0C 0M 0Y 100K.

What am I missing here? When I search online, I mostly find people asking how to get Distiller to NOT create a rich black, so I'm really puzzled.

I'm using Distiller 11.0.07 on a Mac.

Thanks in advance for any help.