  The Rydberg excitation blockade has been at the heart of an impressive array
of recent achievements; however, state-mixing interactions can compromise its
efficiency. When ultracold atoms are excited to Rydberg states near F\"orster
resonance, up to $\sim 50\%$ of atoms can be found in dipole coupled product
states within tens of ns after excitation. There has been disagreement in the
literature regarding the mechanism by which this mixing occurs. We use
state-selective field ionization spectroscopy to measure, on a shot-by-shot
basis, the distribution of Rydberg states populated during narrowband laser
excitation. Our method allows us to both determine the number of additional
Rydberg excitations added by each mixing event, and to quantify the extent to
which state mixing ``breaks'' the blockade. For excitation of ultracold
rubidium atoms to $nD_{5/2}$ states, we find that the mixing is consistent with
a three-body process, except near exact F\"orster resonance.
