Q:

How common are binary stars with only one visible sibling?

Single-lined spectroscopic binaries is a binary system where only one star can be detected.
Are they common enough to contribute noticeable to dark matter?

A:

The luminosity of normal stars is a strongly increasing function of mass. e.g. $L \propto M^3$. If another star is "hidden" in a binary system, then it is of lower mass. So the amount of hidden mass is less than what is seen.
Of course this can be accounted for when estimating the mass present in luminous matter because we know typical binary frequencies and properties. The luminous matter is only about 1% of that required to explain the properties of the universe as a whole and less than 10% of the mass required to explain the dynamics of ours and other galaxies, so even a factor of 2 is not going to solve the problem.
Further, from big bang nucleoynthesis calculations and estimates of the primordial abundances of helium and deuterium, we know the missing mass is not "baryonic". It is not normal matter made if protons and neutrons.
So hidden binary companions are not an important contributor to the dark matter problem

