In general, it's good to keep your metaphysics and epistemology in line; that is, what's going on inside your head should match what's going on in the world. Let's translate the old adage, "Ignorance is bliss," into philosophical terms: if happiness is solely an epistemological issue, then maybe it's best not to seek out certain kinds of knowledge. Is that even possible?
This all stems from an argument I had with a friend today. Imagine a situation like this: I have a bottle of wine in my cellar from '93. I could open it now and it might be fabulous. However, I don't know anything about '93 or the winery, so it might also taste like balsamic vinegar. An idealized conversation:
MNJ: Clearly it's best to leave the bottle corked, as then you won't have to deal with the disappointment of bad wine.
Friend: If you open it now, Eminjay, the worst case is that it's bad and you have to pour the bottle, which is the same outcome of not opening it at all. At least when opened, there is the possibility an enjoyable bottle!
MNJ: The bitter taste might be awful.
Friend: But brief, Eminjay.
MNJ: The mere possibility of the '93 being good is better than the reality of it being bad, even if the wine's not drunk in either case.
Friend: That's obviously wrong.
MNJ: Leave a kiss but in the cup and I'll not drink the wine.
I'll explore three possible frameworks to make "Ignorance is bliss," a viable life philosophy. And I'll fail.
Actually, there is one trivial case: if you really have no idea that things are bad, then you will certainly be happy. Perhaps you're enjoying food at your favorite restaurant, enjoying dinner with a hot date. Should you eat a spinach salad with E. Coli, then you'll remain happy until you (and likely your date) discover this unfortunate fact, when the metaphysical havoc of your GI track takes hold. Black and white. And brown.
Now, let's say that you're a Creationist and you're faced with a mountain of evidence that evolution is true. The evidence points to a metaphysical reality that doesn't suit you well. There are a couple of ways to deal with this problem. The first and weakest idea would be to believe any evidence that supports Creationism and deny any evidence that does not. Instead of being based on ignorance, bliss derived from said method is simply stupid. This may work for some, but one wonders what acrobatics their brains must dance in order to keep reality so separate from fantasy. Especially for issues more difficult to avoid than evolution (e.g. "My wife still loves me"), denial is not viable.
Another tactic might be to ignore as much of that evidence as possible: turn off the TV when you hear something contradictory, read good Creationist magazines, go to a Creationist school, and surround yourself with people who have similar view. This seems completely unsatisfactory. In denying yourself access to certain kinds of knowledge, you are acknowledging that it might be the case that evolution is true. That implicit assumption of possibility ruins happiness for you: though maintain ignorance of direct evidence, you do have some knowledge about evolution, namely that you know that some evidence exists, even if you aren't privy to the details. You don't know how strong (in this case) or how weak (which could be the case in many other situations) the evidence actually is. By refusing to hear evidence, you implicitly give the evidence value, no matter what the actual outcome is. This can't be called happiness, since you will live in constant fear of happening upon some unsettling fact.
Another, very versatile, idea would be to assume that Creationism is true, evidence be damned. Then, rework your framework so that facts that support evolution actually support your revised theory of Creationism. This methodology has the advantage of being very flexible: even new evidence can be adapted into the new theory. However, even this will fail at some point. Once there is a preponderance of evidence, the only way to work Creationism into reality would be to change your framework so drastically that other parts of your happiness completely break. For example, if you're a scientist, you might be stuck with the choice of denying the scientific method in general, which would likely cost your job, or denying the scientific method for Creationism only, which brings you to the same denial in our first case.
I've explored three ways to deal with the existence of evidence: deny it, ignore it, or change your framework so that it becomes truth. Bottom line: I don't know and I'm totally unhappy.