Actually there's not one singular language used by carnival folk, while a lot of them have very similar roots, they diverge drastically enough between troupes that they count as distinct cants. You could theoretically decipher one with knowledge of another but that's more like figuring out French with knowledge of Spanish. You'll get the broad meaning but you may miss the details.

As for blacklist, I was disappointed to find there's isn't an authentic cant, but it is portrayed rather accurately to how smaller troupes developed their dialects.