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NCBI has been transitioning to Accession-Based BLAST.  These changes will not affect your web BLAST searches unless you limit your search with the Entrez Query option. This option is only available if the database that you search is a traditional BLAST database. As of January 2019, the following databases for Protein BLAST on the web (blastp) are accession-based, and will not have the Entrez Query option: Metagenomic proteins or Transcriptome Shotgun Assembly proteins.

To continue limiting your searches with Entrez queries, please consider switching to BLAST+ (command-line tools). You will need to approach as follows:  

1. Conduct your Entrez search independently from your BLAST search by using one of the following:

2. In all cases, download the results of your search as a list of accession numbers.

3. Use the accession lists in BLAST+ for Accession-Based BLAST  with the following two commands  -seqidlist or -negative_seqidlist