  Some ultraluminous X-ray sources in nearby galaxies show soft components
resembling thermal disc emission. Calculations based on blackbody emission then
indicate that the accreting black holes at the centres of these discs have
masses of 100s to 1000s Msun. Establishing the existence of such intermediate
mass black holes is of considerable importance so the assumptions and
approximations lying behind blackbody spectral fits must be examined. We study
here the basic assumption that the thermal emission is well-characterised by a
multi-temperature blackbody spectrum. Since the opacity in the surface layers
of a disc decreases at high energies the emergent spectrum is hardened. We
compute the observed spectra from discs around a non-spinning 1000 Msun black
hole and fit them over the XMM-Newton pn band with multi-colour disc models.
The typical overall spectra hardening factor usually adopted for discs around
stellar mass black hole (assuming an inclination of 60 deg and including
relativistic effects for a non-spinning black hole) is found to be appropriate.
