Actually, fat bloom is like sugar bloom in your chocolate. All it means is that either the fat or the sugar crystallizes together and forms something you can visually see.
Your chocolate SHOULD be shiny, and have a snap to it when you break it.
However, if you have Bloom, it will have a whitish sheen or coating or spottiness to it. It's okay and it doesn't necessarily mean the chocolate has gone bad, although fat bloom is worst than sugar bloom.
Sugar bloom is the crystallization of the sugar crystals and occurs when moisture hits your chocolate. Very common. Very normal. Fat Bloom happens when the chocolate hasn't been made well (tempered correctly by the manufacturer).
How can you tell the difference? With a simple test (from the Amano chocolate website, great summary article here)
One way you can easily check to see if a piece of chocolate has undergone sugar bloom or fat bloom is to lick your finger and touch it to the chocolate. If the dusty appearance disappears, then it is sugar bloom. (The moisture on your finger dissolved the sugar crystals on the chocolate.) If the bloom remains, then it is fat bloom.
And, of course, if you don't want to work up your own personal "fat bloom", the bottom line (pun, totally intended) is to eat really good chocolate but eat it in control.
For more information: Click here to visit Will Clower's website.