  We demonstrate the interaction-induced blockade effect in an ultracold
$^{88}$Sr gas via studying the time dynamics of a two-photon excitation to the
triplet Rydberg series $5\mathrm{s}n\mathrm{s}\, ^3\textrm{S}_1$ for five
different principle quantum numbers $n$ ranging from 19 to 37. By using a
multi-pulse excitation sequence to increase the detection sensitivity we could
identify Rydberg-excitation-induced atom losses as low as $<1\%$. Based on an
optical Bloch equation formalism, treating the Rydberg-Rydberg interaction on a
mean-field level, the van der Waals coefficients are extracted from the
observed dynamics, which agree fairly well with \emph{ab initio} calculations.
