An onion is cut into small pieces with a fleishig knife. Then the pieces of onion are chopped in a food processor that was recently used to proses dairy. The onion is obviously treif, but what is the status of the food processor?
There is a conflict of opinions whether or not a davar charif (sharp food) transfers taste from a meat knife, that it was first cut with, into another [neutral] utensil (see Magen Avraham 451:31) or not (see Even Ha'ozer 96:3). It therefore may be argued that in OP's case the meat taste from the knife became pagum (soiled) and therefore no issur (prohibited food) was transferred into the blender (cf. Rabbi MT Dinkel's 'Davar Charif' pg. 56). Additionally, there is the opinion that a sharp food can only emit absorbed flavor by duchka de'sakina (when pressure is applied) (cf. Pri Megadim, EA 447:32) and a blender may or may not fit this category. If the blender was not used that same day on which the onion was processed there may be different opions at play to be lenient with regards to the bender's status (cf. Dinkel, op. cit., pg. 58).