blas-atlas should set itself the system default blas implementation at the end of the build process if ther is no other one already used. actually the 3.7.11 ebuild tries to do it by using blas-config but it fails, because blas-config is not present in the system (it's not listed in blas-atlas' dependencies). Probably `eselect blas ...` should have been used, as the dependence on eselect suggests.
Dear Alessandro, Thank you very much for your report! This has just been fixed in portage CVS. Thanks, Markus