2003 Chevrolet Trailblazer Service Engine Soon

Obviously you want to find out the exact code but to start with, if the spark plugs are original, which is possible at the mileage you listed, replace them, then see if the problem goes away. Code 300 is for a random cylinder misfire. You may also get a code 301 - 306 indicating which cylinder is misfiring. If it's always the same one or two, you might suspect the injectors too. GM doesn't flow-match them before installing them on the assembly line. After many miles they don't flow the same rate. Some cylinders will be a little lean. That is detected by the oxygen sensors and the Engine Computer responds by adding fuel to all the cylinders on that side of the engine.

The flashing Check Engine light means that too much raw fuel is going into the exhaust system where it is going to overheat and damage the catalytic converter. You should stop the engine right away when that happens while you're driving and the engine is already warmed up. Most commonly that is caused by loss of spark to one or more cylinders. It can also be caused by a leaking fuel pressure regulator which GM did have quite a bit of trouble with, but that typically wouldn't clear up after the engine was running a while.