Hi, this happens to me too (using Windows Desktop app). Therefore, I made this little test:
1. Created new document (Publisher)
2. created a box filled with pure black
3. Created some text (pure black)
4. Double-clicked the "colour-fill-circle" in Publisher
The text appears to be totally black in the colour-square (dot located in the farest left-down corner). Bud the RGB-values are actually (31,35,32) and also in colour triangle one can see that the colour is not pure black.
Also, compared to the black box, the text is not black.
If I export as PDF now, the text will not be totally black (I can check that with the colour pick-tool after import).
If I manually re-drag the dot in the colour square to farest left-down), the colour changes on the text (and in the RGB values, as also in the colour-triangle).
If I export as PDF now, everything seems normal, text is totally black (I can check that with the colour pick-tool after import).
Could this be some kind of bug? I think it might have something to do with your problem anyways.
Best
Mattias