So i was driving my 03 trailblazer with a 4.2 and when i started picking up speed and got to just shy of 3000 rpm my service engine light blinked like 5 times them went away when i lowered speed… what could this possibly be?

 

flashing ses light = engine misfire. this condition can damage the cats so it should be addressed soon.
one option is restricted exhaust.
hammer fist the cats and if it sounds like something is loose inside...the cat is blocking exhaust flow
remove the pre cat 02 sensors from the exhaust. the exhaust gas can exit out the 02 holes...test drive and if the misfire is gone...you have confirmed exhaust restriction.

 

^ +1! Do not ignore a flashing check engine light! It can be extremely expensive to do so!