Thanks for passing by and the question.

So dark matter ignores light as you say, but at the same time bends light due its gravitational interactions which it has in common with its sibling, the standard matter.

In fact, I was a bit sloppy here, for simplifications reasons. Two things are in fact mixed.

• First, dark matter ignores light in the electromagnetic sense. It is totally insensitive to electromagnetism and therefore does not absorb, reflect or emit light.
• Second, dark matter interacts gravitationally, and in this way modifies the structure of spacetime (as any massive object does). This is this modification that impacts the trajectories followed by light when it travels into space.

This second point applies to black holes too. They will modify the structure of spacetime and hence (drastically) impact the trajectory of light traveling into space.

Now let's move on with your question.

By which observation can it be ruled out that dark matter is nothing but a gigantic network of countless standard matter black holes? If those had formed early on in huge quantities and scales, couldn't that explain the inhomogeneities wich we observe in the visible universe too?

This hypothesis has been made, and studied. This is what is called primordial black holes. This assumption is not ruled out, but black holes enough are not sufficient to explain all observations. We indeed need a second source of dark matter in this context. On the other hand, the "no black hole" hypothesis is not rejected (at least "yet").

I hope this answers the question and clarifies the situation. Please let me know.

PS: This actually extends what I answered @agmoore, who I tag so that she could get more information.