Boeing placed well in both the ATF (leading to the F-22, where they joined the team headed by eventual winners Lockheed) and JSF (leading to the F-35, where they received a contract to build the X-32 demonstrator as runners-up) competitions and in both instances submitted designs with a blocker. So it stands to reason that matching the ATF & JSF (i.e. F-22 & F-35) stealth requirements is possible with a blocker, and that the engine performance penalty of a well designed blocker is not so large as to preclude supercruise satisfying the ATF spec.
Boeing ATF design:
Boeing ATF design: