When Windows 10 Technical Preview Build 10061 was released, Microsoft has listed some of the known issues users may face after installing Build 10061. One of them was related to black screen.
There is a known issue where you might see a black screen with only your mouse cursor during login/logout. We have a fix for this coming via WU.
And today Microsoft has pushed the fix via Windows Update. Users may notice this in their WU
Update for Windows Technical Preview April Update for x64-based systems (KB3055415).
The details of KB3055415 can be found here:
The lock screen in Windows 10 Technical Preview Build 10061 might appear as a black screen with a mouse cursor https://support.microsoft.com/en-us/kb/3055415/
So if you are on Build 10061 and having this issue then can check for Windows updates if not yet installed.