  The discovery of the "hidden population", whose size and membership are
unknown, is made possible by assuming that its members are connected in a
social network by their relationships. We explore these groups by a
chain-referral sampling (CRS) method, where participants recommend the people
they know. This leads to the study of a Markov chain on a random graph where
vertices represent individuals and edges connecting any two nodes describe the
relationships between corresponding people. We are interested in the study of
CRS process on the stochastic block model (SBM), which extends the well-known
Erd\"os-R\'enyi graphs to populations partitioned into communities. The SBM
considered here is characterized by a number of vertices $N$, a number of
communities (blocks) $m$, proportion of each community $\pi=(\pi_1,...,\pi_m)$
and a pattern for connection between blocks $P=(\lambda_{kl}/N)_{(k,l) \in
\{1,...,m\}^2}$. In this paper, we give a precise description of the dynamic of
CRS process in discrete time on an SBM. The difficulty lies in handling the
heterogeneity of the graph. We prove that when the population's size is large,
the normalized stochastic process of the referral chain behaves like a
deterministic curve which is the unique solution of a system of ODEs.
