https://www.census.gov/library/stories/state-by-state/maine-population-change-between-census-decade.html

There's no elegant breakdown of the data but the best way to get a sense of it is by choosing the "[race] in combination" drop down at the Census link above for each race/ethnicity. The two largest non-white ones reported with others are Native at almost 2% and Black at about 1%. Total mixed is 4.7% and white in combo is 4.6% which means the vast majority of mixed are white and something else -- makes sense given Northern New England has been the whitest part of the country for a very long time.