I asked the color guy here and got this response:
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Probably the first question is whether the PDF itself contains the rich black or the rich black only appears when printing the PDF. Based on the description I think I lean toward the latter. If that’s correct then it may be an Acrobat color management issue. One way to find out is to place the resulting PDF into a new ID doc and print and see if you get rich black output. If you do, the PDF is bad, and if you don’t, it’s an Acrobat thing. (This assumes your color settings are configured with a CMYK profile policy of “Off” or “Preserve Numbers”.)
The default PDF export presets in ID should not include any profiles for the text, although for PDF/X variants the output intent will imply one. If Acrobat is configured to use that profile and its print dialog is configured in such a way that you are requesting a conversion, you’ll get one, and your text will go 4c. The gradients are specified as DeviceN which would escape the conversion. The tint vs opacity example is strange; if anything I would expect it the other way around, but that’s only based on a guess as to what may be happening.
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Hope this helps. Not my area.