I too did exactly what you described. I suspect it is a cache problem ( that seems to be a not too uncommon theme here - pardon the pun ! ).
I did not experience a blank screen as you describe, but I just found sometimes I had to be patient, and eventually the new theme came good.
Also on one or two occasions I found even though paddygreen was the default theme and was mainly working OK, it sometimes reverted back to blackcandy, but then I just refreshed the browser and back came the paddygreen.
But when i restart the webbrowser, inc the ?flush=1 parameter but i get a completely blank page.
This may be an error. If you open up mysite/_config.php and make sure you have Director::set_environment_type("dev"); then try changing the theme again it might come up with a error rather then a blank page.
thanks for the pointer willr, I've manged to crack what the problem was last night. for some reason when I copied the files into /themes not all the files copied. I have no idea what made made me check the actual files but i did. I recopied and all is well.