Yes, HRT, especially estrogen-based treatments, can darken existing freckles or make dormant freckles easier to see.
Sunscreen, sunscreen, sunscreen.
I sound like my own dermatologist, but I know the risks. (I have had 8 skin cancers removed). Estrogen affects sensitivity to UV light, and, of course, natural aging doesn't help. It is not just freckles either. Other brown patches, we called "age spots" or "liver spots," can also appear.