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Adobe product and version number.
Operating system and version number.
What you were doing when the problem occurred.
Full screenshots of the problem.
Computer hardware, such as CPU, GPU, amount of RAM.
And more importantly, what's your end goal?
What are the Foreground and Background Colours in the Toolbox?
Have you hit D to reset them yet?
Rather than trying to home in on what setting is causing this, I'd be inclined to save yourself the time, and reset the tool. (Left end of Options bar) In fact I usually reset All Tools when stuff like this happens, and it happens to us all.
As far as I know the Filter »Stamp« uses the Fore- and Background Colors, so if they are both black the result the OP mentions would be as expected.
A screenshot would naturally be useful.
Resetting the effect worked! Thanks guys!