Can we just clarify - Blackswift does NOT include PDE (pulsed detonation engine). PDE is part of Darpa's Vulcan program to demonstrate an engine for a full-scale hypersonic vehicle. Vulcan is related to, but separate from, Darpa's Falcon and Blackswift programs.
Blackswift will be powered by a combination of high-Mach turbojets (Rolls-Royce or Williams) and dual-mode ramjet/scramjets (Pratt & Whitney Rocketdyne). The turbojet and scramjet will share the same axisymmetric/inward-turning/three-dimensional/round inlet. A door will divert the flow from the turbojet to the scramjet and vice versa.
Vulcan, meanwhile, is a program to demonstrate an engine - based on an existing, in-production, Mach 2-class fighter engine - that could accelerate a full-scale hypersonic vehicle from zero airspeed to Mach 4-plus and scramjet ingition. In Vulcan, the PDE - either mounted in parallel with or wrapped around the turbojet - would take the place of the ramjet and avoid need for the very expensive development of a full-size high-Mach turbojet.