Ignorance is bliss only when you are ignorant of the fact that you are ignorant. In other words, it is not ignorance which is bliss but rather, absolute ignorance which may be regarded as bliss.
There is no bliss if one is painfully aware that one is ignorant. There is no bliss in complete knowledge. There is no bliss in partial knowledge. There is no bliss in partial ignorance.
Then again, it depends on the kind of knowledge. And it depends on how you look at bliss.
But generally knowledge is a double-edged sword. And furthermore, some (many) things are better off not knowing.
Absolute ignorance huh. To realise that ignorance is bliss is to be at a point so far gone that one can never enjoy ignorance again. In other words, one can never appreciate the bliss of ignorance, for to do she he or she would have to come to terms with his or her ignorance, and in the process lose the very thing which makes ignorance bliss.