I forgot to check back here, sorry. I got shown the software blast2go on seqanswers, which does what I want, but unfortunately is quite slow and cumbersome. I am currently working on a (crappy) python script to do this for me, for now with Cuffdiff output. My approach is:
1) Take only significant hits in isoform_sig.diff and convert it to a bed file taking each TCONS as a gene (one line). 2) use fastaFromBed in the fastx toolkit to create a fasta file. 3) Run that through my script using wwwblast/qblast in biopython to slowly blast against the database of my choice on NCBI.