Part of that's due to its comparative lack of suburbs, though, so unlike in most major cities, whites actually live in the city instead of the suburbs. For example, Boston is 55% white within the city limits, but Greater Boston is closer to 70% white, which isn't far off of Portland's 78%.

Edit: Actually it looks like Greater Boston is more like 80% white. The state of Massachusetts overall is ~88% white, and the Greater Boston area has about 60-65% of MA's population. Even if we assume every single non-white person in MA lives in the Boston area, they'd make up <20% of the metro-area's population.