Can you identify which black breed this hen is?

[NatJ]

My suspicion is that she isn’t laying yet and that my Sussex is laying the creamy eggs.

Have you tried looking at their vents to see which one is laying? There is often a very clear difference between layers (large moist-looking vent) and not-layers (small, dry-looking, puckered vent.)

If you need a comparison, grab a hen you know is laying, and a chicken that you know is not laying. Roosters are good for this--perfect example of a vent that has never laid an egg!