Drupal 7 views Contextual filters with Blocks, default value doesn't work properly , wield
If your first contextual filter is your username filter then it will respond to the first argument passed in the preview textfield, no matter what default value you have specified. Your default value is for when an argument is not passed. So, entering "projects/abc" in the Preview with contextual filters box will pass 'projects' to your username filter, but passing a valid username as the first argument as in "abc/*" will work correctly.

It sounds like you've got it set up correctly to me. Have you tried deploying your block view on an actual page?