Blackfoot is a slur used to describe French in a derogatory way. Possible reference to French-Indian War and/or WW1 and the general sense of French cowardess during wars. Since they had no boots due to the trenches eating them away, when they ran their feet became all covered in mud. Or French born in North Africa.
The word Blackfoot is a racial slur that has been used historically to degrade and dehumanize french. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to french. If you are unsure about whether or not it is appropriate to use the word Blackfoot, it is best to err on the side of caution and avoid using it in any situation.