  Measuring blackbody parameters for objects hotter than a few 10^4K with
optical data alone is common in many astrophysical studies. However this
process is prone to large errors because at those temperatures the optical
bands are mostly sampling the Rayleigh-Jeans tail of the spectrum. Here we
quantify these errors by simulating different blackbodies, sampling them in
various bands with realistic measurement errors, and re-fitting them to
blackbodies using two different methods and two different priors. We find that
when using only optical data, log-uniform priors perform better than uniform
priors. Still, measured temperatures of blackbodies above ~35,000K can be wrong
by ~10,000K, and only lower limits can be obtained for temperatures of
blackbodies hotter than ~50,000K. Bolometric luminosities estimated from
optical-only blackbody fits can be wrong by factors of 3-5. When adding
space-based ultraviolet data, these errors shrink significantly. For when such
data are not available, we provide plots and tables of the distributions of
true temperatures that can result in various measured temperatures. It is
important to take these distributions into account as systematic uncertainties
when fitting hot blackbodies with optical data alone.
