As a number of people have said, blood tests can only determine allergies. A mild-severe allergy may irritate the body enough to bring on a nasty headache or even a migraine. However, when we discuss food triggers that may cause migraine, these are not allergies. Allergies are categorized by an immune response whereas migraine triggers are categorized by, well, you know. The ingredients that most often are headache triggers are nitrates, tyramine, MSG, other preservatives and by-products of many fruits, vegetables and aged or smoked products, etc.
Here's a link to a story that explains Dr. David Buchholz's elimination diet: http://www.npr.org/templates/story/story.php?storyId=5364970 . It goes quite a bit over the top, but certaintly covers just about all your bases in foods to eliminate. For a better list and more of his (very controversial) theories about the triggers of migraine see "Heal your Headache" by Dr. David Buchholz. I don't care for the book much but it's the most comprehensive work on dietary triggers that I've encountered.
EDIT: Oh, here's his list: http://www.npr.org/programs/morning/features/2006/apr/migraine/triggers2.pdf
DX: NDPH, Recovered CRPS
RX: Lamictal, Abilify, Provigil, Clonazepam, Ambien CR, Emsam, Namenda
PRN: Haloperidol, Zyprexa, Lodine, Zofran, Oxycodone, Skelaxin, Diluadad