You imply that there is only one "form" of his face depicted in iconography, but this is not the case. There is wide variation in how he has been depicted going back to the 14th century. Here is a selection of images from the 14th and 15th centuries which are closer to the reconstruction than they are different:
Italy had and still has a pretty diverse makeup - Germanic peoples from the north, North Africans from the south, and Mediterranean peoples who are the crossroads of the two.
Where I live in the torturous mountains of the north of Portugal, people from villages all of five kilometres apart can look radically different. Our nearest village is your fairly standard Iberian phenotype - dark brown to black hair, and tanned looking. Across the valley is a village that took in Jews fleeing the inquisition - and they look Sephardic to this day. Ten kilometers north is a village still named in local dialect “Moorish village”, and lo and behold, the people there look Arabic.
So what was his phenotype? Only going to find that out by sequencing him. He probably had dark skin and hair, but he could have been blonde and pale.
Who knows? He might have been blonde-haired for all we know, after all the Duchy of Spoleto set up by the Lombards was also located in Central Italy. You can get an idea of that past Lombard presence from this map [1]: "
Percentage of Blond Hair in the Italian regions" (notice the green blob East of Naples), which, granted, it may not be 100% scientific but I reckon that it is at least based on some real data.
Later edit: Apparently that map is based on this mid-19th century data sample: Percentages of blond hair in the Italian regions (including Corsica). Data collected by Ridolfo Livi on 1859-1863 lever classes ( "Renato Biasutti - Races and peoples of the Earth - UTET, 1941")
There are many plausible "skins" for Aquinas. He didn't come from relatively homogeneous place like Japan on Iceland, he was Italian. And medieval Central Italy was a massive genetic melting pot of Etruscans, Latins, Greeks, Celts, Germanic people and North Africans. It used to be crossroads of a massive empire once, and was overrun by several invasions of other peoples afterwards.
This is SO catholic (well, or religious in general).