In general, you can use any of the statistics from the server, feed them into a Discrete-Event Subsystem (DESS) from the SimEvents / Ports and Subsystems library, and inside the DESS use any Simulink Math blocks to compute a desired result from this, if you know the formula.
I think for blocking probability you would need the server utilization, and probably the queue capacity. If so, the former can be obtained from the server block, and for the latter, use a Constant block inside the Discrete-Event Subsystem with constant value equal to queue capacity.
Disclaimer: Not sure about the correct formula for blocking probability. You'll have to look that up.