I was wondering whether the peptide sequences that correspond to mass spectra, deposited in spectra databases, can be obtained somewhere?
I am not sure if Mascot is what you are looking for.
Probably not I believe not (though I don't have much familiarity with MS/MS databases). If you can get the spectra and a protein database, you can estimate the protein abundances.
What "blast" has to do with your actual question?
I don't think so.. as I would like to validate whether the spectrum, corresponding to the peptide sequence, is already present in the depository/database, prior to the real mass spec experiment.