Rebmu: 16 chars
en'PNGmkIM![1x1]
If you want it to save to a file based on an argument you pass in, that adds three more chars to the program:
rebmu/args [wrAen'PNGmkIM![1x1]] %my-black-pixel.png
The program is shorthand for the following Rebol, parentheses added for clarity:
write a (encode 'png (make image! [1x1]))
