The 'prefix' is not intended to be the full filename for output -- except in the special case of NIfTI-1 output, which was grafted onto the AFNI prefix mechanism about 10 years after AFNI was crafted.

If you wish to specify that an AFNI .BRIK file be compressed on output, you can do one of the following (via environment variables):

setenv AFNI_COMPRESSOR GZIP
setenv AFNI_AUTOGZIP YES

or, on a one-off basis

3dSomething -DAFNI_COMPRESSOR=GZIP -prefix Elvis ...

I don't plan to adapt the AFNI-wide prefix parsing scheme to fit the scheme that you have tried to use.

As for the message
"++ Output dataset ./blank+orig.BRIK"
being incorrect -- that's an artifact of the way the I/O functions handle compression, which is on-the-fly (through a pipe) and the choice of compression is invisible above the actual output function. To fix this would require a decent amount of tinkering with code that is now nearly 14 years old -- something I don't plan to do just to fix a minor cosmetic issue.