Notes on NCBI's Basic Local Alignment Search Tool (BLAST) software suite:
blastx: blastx is used to search a protein database using a translated nucleotide sequence query.
tblastn: tblastn is used to search a translated nucleotide database using a protein sequence query.
tblastx: tblastx is used to search a translated nucleotide database using a translated nucleotide sequence query.
Ian Korf: BLAST: http://www.amazon.com/dp/0596002998/ref=rdr_ext_tmb
Selecting a different algorith may be the difference between a hit and a "no significant similarity found" result. If no significant similarity is found at first attempt, try several algorithms.