  In this note, we are concerned with dark modes in a class of non-Markovian
open quantum systems. Based on a microscopic model, a time-convoluted linear
quantum stochastic differential equation and an output equation are derived to
describe the system dynamics. The definition of dark modes is given building on
the input-output structure of the system. Then, we present a necessary and
sufficient condition for the existence of dark modes. Also, the problem of dark
mode synthesis via Hamiltonian engineering is constructively solved and an
example is presented to illustrate our results.
