Who might my ancestors be?

This. . . . You can't judge by appearances alone.

You should ask your elders which direction your family immigrated to Shanghai.

If you immigrated to Shanghai from the north, your ancestors should have the blood of the northern minorities in China, or to be precise, the white minorities in the northwest, such as Uighurs and Kazakhs.

If it is in the south, it should be from Southeast Asia.

By the way, you should also look at your family's hair, whether it is straight, curly or wavy. White curly hair, black curly hair (it should not be curly hair), yellow people in the middle.

Judging from your description, "some people say they look like foreigners, but there is still a big difference." The skin is slightly black. Thin but not tall "should be from Southeast Asia, because the northern nationalities in China are whiter and taller. You can look for photos of people in Southeast Asia and Northwest China to see which one is closer to your family. "