  The Stark shift due to blackbody radiation (BBR) is the key factor limiting
the performance of many atomic frequency standards, with the BBR environment
inside the clock apparatus being difficult to characterize at a high level of
precision. Here we demonstrate an in-vacuum radiation shield that furnishes a
uniform, well-characterized BBR environment for the atoms in an ytterbium
optical lattice clock. Operated at room temperature, this shield enables
specification of the BBR environment to a corresponding fractional clock
uncertainty contribution of $5.5 \times 10^{-19}$. Combined with uncertainty in
the atomic response, the total uncertainty of the BBR Stark shift is now
$1\times10^{-18}$. Further operation of the shield at elevated temperatures
enables a direct measure of the BBR shift temperature dependence and
demonstrates consistency between our evaluated BBR environment and the expected
atomic response.
