  We present ZTF20aajnksq (AT2020blt), a fast-fading ($\Delta r=2.4$ mag in
$\Delta t=1.3$ days) red ($g-r\approx0.6$ mag) and luminous ($M_{1626}=-25.9$)
optical transient at $z=2.9$ discovered by the Zwicky Transient Facility (ZTF).
AT2020blt shares several features in common with afterglows to long-duration
gamma-ray bursts (GRBs): (1) an optical light curve well-described by a broken
power-law with a break at $t_\mathrm{j}=1$ day (observer-frame); (2) a luminous
$(L_X = 10^{46}$ $\mathrm{erg}$ $\mathrm{s}^{-1})$ X-ray counterpart; and (3)
luminous ($L_\nu = 4 \times 10^{31}$ $\mathrm{erg}$ $\mathrm{sec}^{-1}$
$\mathrm{Hz}^{-1}$ at 10 GHz) radio emission. However, no GRB was detected in
the 0.74d between the last ZTF non-detection ($r > 20.64$) and the first ZTF
detection ($r = 19.57$), with an upper limit on the isotropic-equivalent
gamma-ray energy release of $E_{\gamma,\mathrm{iso}} < 7 \times 10^{52}$ erg.
AT2020blt is thus the third afterglow-like transient discovered without a
detected GRB counterpart (after PTF11agg and ZTF19abvizsw) and the second
(after ZTF19abvizsw) with a redshift measurement. We conclude that the
properties of AT2020blt are consistent with a classical (initial Lorentz factor
$\Gamma_0 \gtrsim 100$) on-axis GRB that was missed by high-energy satellites.
Furthermore, by estimating the rate of transients with light curves similar to
that of AT2020blt in ZTF high-cadence data, we agree with previous results that
there is no evidence for an afterglow-like phenomenon that is significantly
more common than classical GRBs. We conclude by discussing the status and
future of fast-transient searches in wide-field high-cadence optical surveys.
