The Census will not change anything. Nielsen, and thus the radio industry, updates population data annually based on a combination of the annual American Community Survey (from the Census Bureau) and Claritas (America's foremost demographer).
Overall, the Boston MSA (there is no "Eastern MA" market) is 70% ethnic, with (rounded) 8% African American, 9% Asian and 12% Hispanic. None of those groups behaves monolitically in its radio listening. Asian listeners will be found to be dispersed among many stations of all kinds. Hispanics will listen to everything from Country to r&b to CHR and Classic Hits. Blacks will listen to many things besides Urban stations.
WJMN is a rhythmic station, not an Urban station. It programs to the market, not to a specific ethnicity. It depends on a coalition audience. While it is true that its 25-54 numbers are lower than ever and revenue is off by about 35% over the last five years assuming that the station is, at present, targeting only African Americans is not a correct assumption. For the moment, they are targeting a coalition audience based on a music style with a roughly 50% current and 50% recurrent mix per my look at the most recent 7 days worth of spins.
There is only one NH county in the MSA, Hillsborough, and that county is only 5% of the total MSA population. WJMN is not taking that area into account at all.