Why do BWB designs typically put the engines in the rear?
The airflow over the top of a BWB accelerates as it would do over the top of any airfoil, and by the time it reaches the wing's trailing edge, its speed has slowed back down, so any engines fitted at the back would see no advantage, from that point of view (although they might indeed have a beneficial effect on the wing's boundary layer).

There are at least two non-aerodynamic reasons for putting the engines at the rear of a BWB. The idea is for BWBs to be more environment-friendly than traditional jetliners, in terms of higher fuel efficiency and lower noise emissions.

In general, the fuel efficiency of a turbofan increases with the engine's diameter (compare the engines of the new A320 neo vs the old ceo), and you would expect a future BWB to have even larger engines than today's ones, which would be difficult to fit under an already thick wing.

Secondly, the engines at the back are also significantly quieter than under the wings, since the body of the aircraft shields people on the ground from the noise produced by the air intake (a major portion of the total noise emitted by a jet aircraft).