There is indeed a difference between reflection and absorption, and it is true that reflection is the primary mechanism for EMI shielding with aluminum foil, but absorption also tends to increase at higher frequencies due to the skin effect, so it may not be accurate to simply contrast aluminum as a pure reflector and shielding paints as primarily absorbing.
Reflection losses are those which are reflected from the external side of the shielding material. Absorption losses are those which are absorbed inside the shielding material itself. Internal reflection losses also occur inside the shielding material itself. Highly conductive materials should generally be good for frequencies between AM radio and UHF microwaves.
Last time I looked into shielding paints, one seller claimed that carbon is a good absorber of RF and that 10% of the shielding effect was due to absorption, which does not sound like much to me, but I guess it would be better than nothing if EMI bouncing around inside the shielded room and concentrating in certain areas is a problem.
As far as I know (I am certainly no expert on the issue), there is nothing really special about carbon-based shielding paints, other than being more convenient and aesthetic than metal foil. Although conductivity is related to reflective characteristics, shielding does not necessarily have to be a very good conductor to work at all. On the other hand, the relatively high resistance of some of these carbon-based paints and metalized plastics might limit their effectiveness.
I would be interested in any studies done on comparisons between carbon-based shielding and common metal shielding, but I would be surprised if thin coats of carbon-based paints were actually better than copper sheets or even aluminum foil.
Different materials demonstrate differences in their shielding properties, which are also frequency dependent, so the best shielding for the job also depends on which frequencies are causing the most problem. I think I mentioned in an earlier post that a reliable broad-spectrum shield for EMI sensitivity would be complicated and/or expensive.
Regarding problems with EMI bouncing around inside a shield constructed of aluminum foil. Although there would still be some greatly attenuated EMI which managed to get through the shielding and would then be reflected inside the shielded room, it sounds like the main problem (I could be wrong) is external EMI getting in through gaps and seams in the shielding.
A distance of 1/20th the wavelength of the EMF is a commonly used guideline for engineers to determine the maximum size of any apertures in the EMI shielding to maintain at least -20db attenuation through those apertures. So for example, the wavelength of 2.4GHz transmissions is about 12cm, so any gaps and seams approaching roughly a centimeter or more in size could be increasingly problematic, particularly for areas closer to the breach. The aluminum foil would also reflect internal sources of EMI, but that does not seem to be what you are talking about.
If materials with free electrons can make good EMI reflectors, and materials with electric and/or magnetic dipoles can make good EMI absorbers, perhaps magnetic metals i.e. those with high permeability should be good at "absorbing" EMI (but more expensive), and are used for shielding low frequency magnetic radiation too, a weakspot for the metals commonly used for EMI shielding. I read about conductive polymers showing future promise for absorption because of their dielectric properties.
I do not have a problem with EMI sensitivity so it is not something I have had to spend much time looking into. It seems difficult to find much good information on the internet, so it might be worth consulting EE text books. Purchasing personal shielding for EMI-S sort of reminds me of purchasing health supplements: I would be weary of taking any anecdotes and manufacturer claims of superiority at face value without further research and experimentation with quality EMF meters.
