It may be worth noting that the contrast being expressed is between Hamlet and his rock-steady friend Horatio. Hamlet is enlisting Horatio's aid in discovering whether King Claudius will react in a self-incriminating way to the play (play-within-the-play) that is about to be performed for the court. Hamlet at this point is plagued equally by suspicion and doubt, and is telling his friend that he needs to rely on a steadier judgment than his own. When he says
And blessed are those
Whose blood and judgment are so well commingled,
That they are not a pipe for Fortune’s finger
he means someone not as passionate and over-wrought as he is. To put it in plainer English:
Look, Horatio, I'm quite knocked about by my passions so that I can't rely on my own judgment. Do me a favor and let me rely on your good sense and judgment, for though I am flitting about in fits of passion, which may make me deceive myself, you are a rock on whose judgment I can depend.
Sounds better the way Shakespeare says it, of course, but that's what he means.