What climate do you live in? Looks like it could be frost damage.

There's also a false melanose form of citrus blackspot that could be the culprit.

I'm just throwing that out there for someone else to help you.

Thank you! We are in Los Angeles. So it's not frost :(

Maybe red citrus scale.

There are several types of Melanose that attacks citrus. Your fruit has Speck Melanose caused by the fungus Disportlhe citri. All species of citrus are susceptible to melanose, but grapefruit and lemons tend to be more seriously affected than the others. The injury to fruit rind caused by speck melanose remains superficial. The fruit is still very eatable. Check your tree for any dead wood, and remove it.

wow. Thank you!

Good call, Silica. Melanose would not have been my first guess in the Los Angeles climate; but you could be right.