From a NYT article about "La Marseillaise." This is, in France, The Year of the Marseillaise, and there was a big conference about it, which much cogitation about the meaning of the words, which were written in 1792, during the French Revolution.
You can read the lyrics in French and English here. The "impure blood" part is:
To arms, citizens,Also at that link (to Wikipedia), we see:
Form your battalions,
Let's march, let's march!
Let an impure blood
Soak our fields!
The English philosopher and reformer Jeremy Bentham, who was declared an honorary citizen of France in 1791 in acknowledgement of his sympathies for the ideals of the French Revolution, was not enamoured of La Marseillaise. Contrasting its qualities with the "beauty" and "simplicity" of "God Save the King", he wrote in 1796:
The War whoop of anarchy, the Marseillais Hymn, is to my ear, I must confess, independently of all moral association, a most dismal, flat, and unpleasing ditty: and to any ear it is at any rate a long winded and complicated one. In the instance of a melody so mischievous in its application, it is a fortunate incident, if, in itself, it should be doomed neither in point of universality, nor permanence, to gain equal hold on the affections of the people.