Is the Oort cloud blocking a substantial amount of light in the visual spectra, making it harder for observers seeing outside the solar system?
I don't have a figure for the total area subtended by the objects in the Oort cloud, and I think it would be very difficult to get even within an order of magnitude estimate for this, since we don't know a lot about the Oort cloud.
However there is a simple answer to your question. All we have to do is ask how often we see stars occluded by objects in the Oort cloud, and the answer is never. That tells us that the amount of light blocked by the Oort cloud is insignificant.
That last rezponse had flawed logic. Just because we never see an Oort object eclipsing a star does not mean the perspective from outside our solar system is the same. Rather it is like we live in a spherical fish bowl that is not perfectly clear because of tiny dots drawn its surface. the sphere of stars is vastly larger than our tiny fishbowl.From a sufficient distance a powerful telescope looking back towards the solar system would see a diffuse ring around the system (the edge of the Oort fishbowl). The amount of light diffused by our Oort clou would be significant enough to alter the perceived spectral lines of the sun, distorting the perceived chemical makeup. An alien with a telescope might think our sun has water and methane and other minerals it should not have due to this distortion. It is the presence of such elements and compounds that tells us which stars have oort clouds since they cannot survive in the stellar furnaces.
Now, the scattering of light might be in the decimal region, but considering the apparent diameter of a star in relation to the size of an Oort-like sphere, a sensitive space-based telescope could detect the presence of the cloud by the inconsistency between the mass and diameter of the star and the amount of light expected and received. Not sure if Hubble is good enough. But the point is that "significance" itself is relative to not just measurement but also WHAT one is measuring.