Places less culturally diverse are far more racist than culturally diverse places like America, but less explicitly so because there isn't as much a chance of an incident.
You're basically right. Race in Portugal is tabu. Politicians are afraid to talk about it and people in general are aware of the lack of integration in Portugal but don't wanna talk about it. We don't even know the real number of Afro-Portuguese residents. The official estimate is around 150,000 but some say the real number is around a 1,000,000. If this is true, we don't know as the Census doesn't have questions about race, Portugal has almost the same share of black population as the US. And its not like there's few incidents but people just don't wanna talk about it. It sad actually.