  We study the unconventional photon blockade, recently proposed for a
coupled-cavity system, in presence of input and output quantum fields. Mixing
of the input or output channels still allows strong photon antibunching of the
output field, but for optimal values of the system parameters that differ
substantially from those that maximize antibunching of the intracavity field.
This result shows that the specific input-output geometry in a photonic system
determines the optimal design in view of single-photon device operation. We
provide a compact analytical formula that allows finding the optimal parameters
for each specific system geometry.
