They looked like fire to me. Also SOUNDED like fire. Also, a couple times they melted rocks.

The Kyuubi's chakra attacks are actually usually black with red highlights. The fire attacks were just straight up orange flames
the kyuubi's chakra burns away naruto's skin and everything else it touches
during the fight against Oro he used his fire-lazer too, so its nothing new

the kyuubi's raw chakra seems to be that way.
it always looked like magma anyway, even (or especially) during naruto's 1-tailed form