How can I get the viscous coefficient of friction B for a DC motor?
The 'Mechanic' block stands for the mechanical part of the system, which is described by the Newton's second law for rotation:
$$
J\dot \omega=\tau,
$$
where tau is the overall torque. If we split it into the torque of viscous friction and the external torque, we get
$$
J\dot \omega=\tau_{ext}-B \omega
$$
or
$$
J\dot \omega+B \omega=\tau_{ext},
$$
where B is a viscous damping coefficient.
Computing the Laplace transforms gives us
$$
(Js+B)\Omega= T_{ext}
$$
or
$$
\Omega=\frac1{Js+B} T_{ext}.
$$
Thus, B from the 'mechanic' block is nothing but a viscous damping coefficient.