...
To remove client area branding, edit the file app/views/client/default/structure.pdt and look toward the bottom of the file for a line that starts with "<p>Powered by" and remove the line.
Admin Area
It's usually not necessary to remove the branding in the admin area as no clients will see it. However, to remove admin area branding, edit the file app/views/admin/default/structure.pdt and look toward the bottom of the file for a line that begins with "Blesta © 2007-" and edit as desired.