At first listen, our ears would tell us that the on board mic is “louder” than your Blue mic. But in actuality, this isn’t the case. The reason for this is “perceived loudness”.
When discussing the nature of human hearing, you find out that humans hear on a curve. That means that, within the frequency spectrum, humans perceive certain frequencies or “pitches” as being louder than others, even if they’re played at the exact same volume. These frequencies that we perceive as being “louder” are around the 1kHz to 6kHz frequency range. (A team of scientists researched this phenomenon in the 1930’s. You can read more about that here if you wish: http://en.wikipedia.org/wiki/Fletcher%E2%80%93Munson_curves
So how does this relate to our microphones? Well, as you can imagine, the larger the diaphragm or “ear” of the microphone, the more it can “hear” or pick up. Blue Microphones specializes in creating large diaphragm microphones, which are capable of picking up a much broader range of frequencies or “pitches”. Most consumer microphones, such as the on board mic of a computer, are built using extremely small diaphragms. This means that they are only capable of picking up the smaller frequencies in the frequency spectrum. And as it turns out, those very frequencies, around 1kHz to 6KHz roughly, are the ones that human beings perceive as being “louder”.
This doesn’t mean you are unfounded in your observation that the on board mic of your computer is seemingly louder. This is a common reaction that many people have when experiencing more accurately captured sound for the first time.