|Readers blood orange|
A. To begin with we have to mention a couple facts about citrus in general and Pigmented (Blood) Oranges in particular . . !. Citrus set fruits right after blooming and the small fruits grow to a certain size and stay at that size all summer before beginning their 'sizing' phase in September . . . All summer the small fruits have to hang on the branches exposed to the sun, wind, low humidity and heat potentially causing the rind on the more exposed fuits to get sun scalded . . .Then during the sizing phase the sun scalded areas of the rind have lost much of the suppleness and elasticity to expand and they often split . .. One early symptom can be the area that is sun scalded will color up earlier. . . (As can be seen by the yellowing around the split.)
Of all the oranges, the Pigmented are notorious for having the least densely foliaged canopy; and of the Pigmented Oranges the Moro variety is the most notorious for sporting the least dense canopy . . That said the fruits during the summer may be more vulnerable to sun scald simply from the fewer leaves . . .With maturity the tree gets fuller and fuller and the sun scald drops off appreciable . . .
So, this is sunscald from earlier in the summer expressing itself during the sizing phase . . . Now I know some will want to lay a guilt trip on you about poor watering or poor fertilizing or something else but that is rarely the case . . And with your description of the tree looking healthy that probably isn't the case at all and just the sunscald showing up . .
If, however you are into self blame and want to feel bad then: 'It's all your fault." . .Just kidding .
It will happen once in a while and the best thing you can do is keep the tree as healthy and well foliaged as possible to help shade and thus mitigating the potential for sun scald . . .