The lack of an article indicates that it's a mass noun, meaning that you're referring to the meat, not the animal. However, the lack of partitive case in the Finnish sentence means that it's referring to the animal. Your suggestion would be correct if the Finnish sentence were: "mustaa kanaa; mustat kanat"
I don't know. Now that I think about it, I reckon that's more accurate because the indefinite plural form "black chickens" corresponds more closely to the partitive plural form "mustia kanoja", whereas the definite plural form "the black chickens" is more similar to the nominative plural form "mustat kanat".