  We show that optical spectroscopy of Rydberg states can provide accurate {\em
in situ} thermometry at room-temperature. Transitions from a metastable state
to Rydberg states with principal quantum numbers of 25 to 30 have 200 times
larger fractional frequency sensitivities to blackbody radiation than the
Strontium clock transition. We demonstrate that magic wavelength lattices exist
for both Strontium and Ytterbium transitions between the metastable and Rydberg
states. Frequency measurements of Rydberg transitions with $10^{-16}$ accuracy
provide $10 \, \mathrm{mK}$ resolution and yield a blackbody uncertainty for
the clock transition of $10^{-18}$.
