Blastclust Ignores The Multithreading Argument (-A)

I'm trying to run:

blastclust -a 48 -p T -d ./uniprot/2011_04/all.fasta  -v progress

I have 48 cores on the server thus -a 48 argument, but only 100% CPU (1 core) is used.

I have Blast2 version 2.2.21 (standard for Debian 6).

This seems to be a bug because parallelism is clearly requested and at least a warning message should be show when proceeding on a single core.

Does anyone know how to workaround this?