  The stars in the Magellanic Clouds with the largest degree of obscuration are
used to probe the highly uncertain physics of stars in the asymptotic giant
branch (AGB) phase of evolution. Carbon stars in particular, provide key
information on the amount of third dredge-up (TDU) and mass loss. We use two
independent stellar evolution codes to test how a different treatment of the
physics affects the evolution on the AGB. The output from the two codes are
used to determine the rates of dust formation in the circumstellar envelope,
where the method used to determine the dust is the same for each case. The
stars with the largest degree of obscuration in the LMC and SMC are identified
as the progeny of objects of initial mass $2.5-3~M_{\odot}$ and $\sim
1.5~M_{\odot}$, respectively. This difference in mass is motivated by the
difference in the star formation histories of the two galaxies, and offers a
simple explanation of the redder infrared colours of C-stars in the LMC
compared to their counterparts in the SMC. The comparison with the Spitzer
colours of C-rich AGB stars in the SMC shows that a minimum surface carbon mass
fraction $X(C) \sim 5\times 10^{-3}$ must have been reached by stars of initial
mass around $1.5~M_{\odot}$. Our results confirm the necessity of adopting
low-temperature opacities in stellar evolutionary models of AGB stars. These
opacities allow the stars to obtain mass-loss rates high enough ($\gtrsim
10^{-4}M_{\odot}/yr$) to produce the amount of dust needed to reproduce the
Spitzer colours
