La Marseillaise was written in 1792 by Claude Joseph Rouget de Lisle and the "impure blood" clearly refers to the enemies of France. Take a look at the lyrics immediately following the controversial section.

Here's a translation from Wikipedia:

"Let an impure blood

Water our furrows!

What does this horde of slaves

Of traitors and conspiring kings want?

For whom have these vile chains

These irons, been long prepared? (repeated)

Frenchmen, for us, ah! What outrage

What furious action it must arouse!

It is for us they dare plan

A return to the old slavery!

What! Foreign cohorts!

Would make the law in our homes!

What! These mercenary phalanxes

…(later)

Frenchmen, as magnanimous warriors,

Bear or hold back your blows!

Spare those sorry victims,

For regretfully arming against us (repeated)"

Okay so Frenchmen are magnaminous, valorous warriors. The enemy are the slaves of tyrants. And we're meant to believe the impure blood in a song written by a French revolutionary era freemason refers to the third estate? Seems unlikely to me.