A new WU update is available for Windows 10 Build 10061. The update KB3055415 fixes the Black Screen issue after login. Read the details below.
This update fixes the following issue,
When you sign out of Windows 10 Technical Preview Build 10061 and return to the lock screen, you might see only a black screen with a responsive mouse cursor. Pressing the Alt+Tab keys or the Ctrl+Alt+Del keys has no visible effect.