Many video upload sites will convert the video after you've uploaded it, that's why it's useful to get the best quality encode possible beforehand limiting the damage done to image quality by their subsequent conversion.
Camtasia probably use some other H.264 encoder, although it may be able to use x264 but I'm not familiar with the Camtasia software. If possible you could also edit the video and output in a lossless format then feed that into an x264 encoder.
It might might be fine through Bllip.tv though personally the best quality video I've always seen is on
Vimeo.com as they don't always convert if the video is pre-converted, they have a guide for Camtasia in their
tutorial section (bottom right hand corner).