I have noticed that whenI hit a P block, a bunch of blue coins appear on the screen.
I have never been able to collect them all. Does anything in particular happen if I do? And if not, what makes the blue coins special?
I found this here:
They reappear in New Super Mario Bros. Wii with the same function as in New Super Mario Bros.. But when the player collects them all in a level before they disappear, the audience cheers. They are often arranged in patterns. Blue Coins also appear in New Super Mario Bros. 2.
EDIT:
I found that collecting all of them normally results in an extra power-up. Also, occasionally the trail of blue coins will lead to a hidden '?' block with a 1-up mushroom!
...besides the obvious fact that the blue coins are extra coins as well
Once you manage to collect every blue coin, a one-up-mushroom appeared and tries to "run" away. If you catch it then you get an extra life. However, if you don't collect all the coins, it is obvious that they are extra coins as well. :)
Nothing, other than the extra coins and an applause. You can get item rewards for collecting eight red coins though.