What do blue star coins mean?
The blue star coins simply signify that you've already collected the star coin before, similar to how power stars become clear and translucent in other Mario games.
The coins are used to purchase things in the shop, so there is only a finite number of them -- 3 on each level -- and once collected successfully (as in, you collect it and then complete the level) will forever after be replaced with the blue star coins (which are identical in terms of points and location).