  A selection of 17 stars in the Sloan Digital Sky Survey, previously
identified as DC class white dwarfs (WDs), has been reported to show spectra
very close to blackbody radiation in the wavelength range from ultraviolet to
infrared. Due to the absence of lines and other details in their spectra, the
surface gravity of these objects has not been previously well constrained and
their effective temperatures have been determined by fits to the continuum
spectrum using pure helium atmosphere models. We compute model atmospheres with
pure helium and H/He mixtures and use Gaia DR2 parallaxes available for 16 out
of the 17 selected stars to analyze their physical properties. We find that the
atmospheres of the selected stars are very probably contaminated with a trace
amount of hydrogen as $-6 \leq \log{N_H/N_{He}} \leq -5.4$. For the 16 stars
with Gaia parallaxes, we calculate a mean stellar mass $0.606\pm
0.076\,M_\odot$, which represents typical mass values and surface gravities
($7.8<\log g<8.3$) for WDs.
