I'm not sure how they use it in Mo land, but here in the real world it means when something you think is bad happens, but then something good comes out of it. One example from my life is 8 years ago I ruptured my Achillies, and was forced to go to the doctor(had not been to one in 10 years), while being treated he discovered I had severe high blood pressure, which I now have under control, so rupturing my achillies was a "blessing in disguise".
One of the most alarming uses of this term was something I read years ago by some twisted Evangelical who said that WWII was a blessing in disguise because if it had not been for the Holocaust, the State of Israel would not have been possible.
...which I guess makes Adolf the father of modern Israel? Ouch- I just hurt my brain.
The phrase wouldn't fit so well now, due to me no longer thinking that there is a master of the universe dispensing favors to his favorites. But when I did think that there was such things as a blessing, this is how the phrase worked.
A blessing in disguise is when something happens that you don't like, but later it's something that you are glad happened.
For instance, when my divorce happened it was the worst thing that ever happened to me in my life. A few years later I was so glad that it happened. I would have stayed with her forever out of obligation and not being a quitter. It ended up being about the best thing ever. So I would call it a blessing in disguise. It seemed like a bad thing at the time, but later was a great thing.
If I still believed there was such a thing as blessings, I'd still call it a blessing in disguise.