  A normal metal source reservoir can load two electrons onto a double quantum
dot in the spin-triplet configuration. We show that if the drain lead of the
dot is a spin-singlet superconductor, these electrons cannot form a Cooper pair
and are blockaded on the double dot. We call this phenomenon Andreev blockade
because it arises due to suppressed Andreev reflections. We identify transport
characteristics unique to Andreev blockade. Most significantly, it occurs for
any occupation of the dot adjacent to the superconductor, in contrast with the
well-studied Pauli blockade which requires odd occupations. Andreev blockade is
lifted if quasiparticles are allowed to enter the superconducting lead, but it
should be observable in the hard gap superconductor-semiconductor devices. A
recent experiment tests this model and finds support for several predictions
made here~[P. Zhang, H. Wu, J. Chen, S. A. Khan, P. Krogstrup, D. Pekker, and
S. M. Frolov, arXiv:2102.03283 (2021)]. Andreev blockade should be considered
in the design of topological quantum circuits, hybrid quantum bits and quantum
emulators.
