Assuming Planck's law still holds it should start absorbing radiation instead.
That said Planck's law derives from Gibb's distribution and applying that rule to it suggests it immediately radiates all energy, until it stops having a negative temperature so I'm not sure if negative temperature is compatible with thermal radiation.
That said Planck's law derives from Gibb's distribution and applying that rule to it suggests it immediately radiates all energy, until it stops having a negative temperature so I'm not sure if negative temperature is compatible with thermal radiation.