It is a possibility that blasto may be infecting the dog but it is a better possibility that the blasto did not go because it needs to be hit with a combinational drug protocol which is very typical of blasto. Flagyl as you know is completely ineffective by itself. There are 10 known subtypes of blasto in which each may require a different approach of drug therapy obviously depending upon the subtype you have.
Badbugs.org has a list of combinational drug therapies with testimonies and I have a forum at http://pptu.lefora.com
that might be helpful as well.