Are you running BLAST locally, using BLAST+ executables and preformatted databases that you downloaded from NCBI BLAST FTP directory?
These databases have taxonomic information embedded into each sequence entry. Hence, in BLAST+, you can limit your searches to the organism(s) or taxonomic group(s) of your interest.
To add the limit, use the -taxids flag to your command line. Follow with the taxonomy ID(s)* where you separate multiple IDs with commas.  
In the following command line example, blastn search is limited to genus Escherichia (taxid 561) and to genus Salmonella (taxid 590): 

The taxonomy limit works for all flavors of blast: nucleotide (blastn), protein (blastp), and translated searches (blastx, tblastn, and tblastx).

 

Where can you learn more?

BLAST documentation:

• using BLAST+ executables and NCBI BLAST databases.

*To find the taxonomy ID for your organism you can search the legacy Taxonomy database or the new Taxonomy site in NCBI Datasets.