It’s perfectly fine if the center of your blackened mahi is still a bit pink in the middle. I prefer mine to be medium-rare, so I often end up with a pink center. As a note, the color of your mahi depends on the type of fish that you buy — some varieties are more pink than beige, and vice versa.  Using your food thermometer to determine whether your blackened mahi has an internal temperature of at least 137 degrees F (which means it’s safe to eat) is your best bet.