As the article points out, there is a window between 20 to 100 solar masses available for black hole dark matter to evade the microlensing constraint mentioned in Sean Lakes's answer. Now, the possibility of primordial black hole dark matter has traditionally been considered for very small black hole masses. The idea was that after the big bang small mass black holes could have formed. Black hole masses that are very light, the mass of the Earth or lighter, would be strong gamma ray emitters via the Hawking process. If most of the dark matter mass were in the form of such light black holes, then that would inconsistent with the observed gamma ray background, so we can rule that out.
Larger mass black holes are ruled out via the microlensing constraint, but this leaves open the window between 20 and 100 solar masses. The question is then how such black holes could have formed. You cannot start out with the conventional low mass primordial black holes to get most of the dark matter mass in the 20 to 100 solar mass window via merger processes. But there are possible mechanisms to directly form heavy black holes during the inflationary era, see e.g. this article.