What actually is Blocking flow problem?
A blocking $s$-$t$ flow is a flow whose residual network (consisting of all edges not saturated by the flow) contains no $s$-$t$ path. Stated differently, a blocking flow is a flow which, for every $s$-$t$ path, saturates at least one edge. Equivalently, a blocking flow is a flow which, for every simple $s$-$t$ path, saturates at least one edge.

The blocking flow problem is to find a blocking flow in a flow network. Such a flow exists since a maximum flow is always a blocking flow (but not vice versa).

The concept of blocking flow is used in Dinitz's maximum flow algorithm.