I believe I understand where you are coming from. Consider that most cliches are generalizations, and serve little purpose, except to justify. This one is honestly an oxymoron.
Ignorance really can't be blissful because it is the pure basis of all fear. What causes more stress than the unknown when encountered or going into a new situation unprepared? The phrase you refer too is a rationale for avoidance, for halting one's thinking at a "safe" point. I suppose this would be fine, if the rest of one's existence wasn't progressing forward without them.
The more we learn and are continually open to doing so, the more we close in on "bliss" (1. Extreme happiness; ecstasy. 2. The ecstasy of salvation; spiritual joy) which in our reality is probably unattainable. This I fervently believe.