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?
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.