The accretion rate is far too small to make much difference to Galactic black holes, but how could this be distinguished from the accretion of normal, baryonic matter in any case?
In fact it is easier for black holes to accrete normal matter, since it is easier for such matter to lose its angular momentum, via friction in an accretion disk, and be able to drop into the black hole.
The effective cross-section for the accretion of non-interacting dark matter is determined by an effective geometric size for the black hole, which will be just dependent on its mass and the speed with which it moves relative to the dark matter. This is the so-called "Hoyle-Lyttleton radius" given by
$$R_{\rm HL} = \frac{2GM}{v^2}, $$
where $M$ is the black hole mass and $v$ is its speed with respect to the dark matter background.
The accretion rate is then just
$$\frac{dM}{dt} = \pi R_{\rm HL}^2 \rho v, $$
where $\rho$ is the density of the dark matter.
For Galactic black holes we might assume $M=10M_\odot$, a speed with respect to the Galactic dark matter of 250 km/s (if it is in orbit around the Galaxy at a similar position to the Sun) and $\rho \simeq 0.01 M_\odot$/pc$^3$ at the Sun's position. Putting the numbers in, we find $R_{\rm HL}= 4.3\times 10^{10}$ m (about 0.28 au) and a mass accretion rate of $10^{-17} M_\odot$/year.
Thus, even over the $10^{10}$ year life of the Galaxy, a stellar black hole increases its mass by a neglible amount due to the accretion of dark matter.
