Is the universe macroscopically transparent to CMB? Is the fraction intercepted by stars and dust so tiny that it doesn't have a correction factor? Background
The question Why do some electromagnetic waves continue travelling while others disappear? is interesting, and in addition to the answer there I started to write:

This is a supplementary answer to @ConnorGarcia's answer using the example in the question. It's not very accurate but it does address part of the question:

Cosmic background radiation emitted when the Universe was very young still exists. But my wifi signal seems to disappear a short distance from my apartment. Why?

Imagine walking around in the middle of a city with a receiver that can pull in 900 MHz to 5 GHz. It will tell you that you are in a &quot;soup&quot; of radio photons; no matter where you go inside the city you'll...

and then realized I didn't know what I was talking about.
Question
So instead I'd like to ask about the following:
I think that CMB photons are believed to be &quot;pristine photons&quot; from soon after the big bang rather than absorbed and re-emitted and thermalized, and I seem to remember recently reading an answer here or in Physics SE saying that a measured CMB spectra (in any given direction) is fit with a Planck distribution with unity emissivity; there was no &quot;fudge&quot; or scaling factor in front for the absolute spectral radiance measured.
But now I can't find that answer anywhere!
Together those suggest to me that macroscopically the universe is essentially transparent to CMB, and the fraction intercepted by stars and dust between us and it is so tiny that it doesn't require a correction factor.
Question:  Have I got this right?
For the most part, the CMB photons travel directly to our telescopes from the surface of last scattering.  Some corrections need to be made to determine the blackbody nature of the spectrum, but they are not corrections for absorption of the photons.
The two main corrections are shown clearly by this COBE image sequence:

(from here).
First, the motion of us as observers, relative to the rest frame of the CMB, distorts its single-temperature blackbody shape, making it appear hotter in one direction and cooler in the opposite direction.
Once this is removed (by a sky-position-dependent frequency shift), the foreground emission of the Milky Way becomes apparent (middle panel), and that must be modeled and subtracted. The dust in our Galaxy is on average hotter than the CMB, so its contribution can be measured well by observing over a range of wavelengths, and of course it is most heavily concentrated toward the Galactic plane.
After that is subtracted, the remaining emission (bottom panel) is that of a blackbody to very high precision, with of course the remaining structure that gives us so much interesting information about the early universe.
Here is part of the key concluding paragraph from the Mather et al. 1992 paper on measuring the spectrum:

The FIRAS spectrum of the cosmic microwave background radiation  agrees with a blackbody spectrum to high accuracy.  The CMBR spectrum is the result of fitting a model including a dipole and a dust map derived from 240 micron data, excluding a region around the Galactic center. The final temperature is 2.726 +/- 0.010 K (95% CL systematic), where the error is dominated by our estimate of the thermometry errors.

So they are shifting the emission and subtracting the foreground emission, but not applying any overall correction for absorption of the emission.