Cannot blast database created with blastdb_aliastool: BLAST Database error: No alias or index file found
The problem was that the database must be specified without suffix, i.e. blastn -db testdb as opposed to blastn -db testdb.nal.

By the way, it produces exactly the same output as

blastn -db "FAM1079.ffn FAM3228.ffn FAM6161.ffn FAM19036.ffn" -query query_single_nucl.fasta -outfmt 6


and the performance is not better.