There are many blue coins that cannot be found with specific levels, they are hidden around Delfino Plaza itself.
Fruit Lady Island
Just offshore there is a small island with a palm tree and a Pianta lady on it. Go to the boathouse in Delfino Plaza (where the portal to Ricco Harbor is) and look out to sea and you'll notice the tiny island. Swim out there and talk to the lady. She'll ask you for 3 coconuts, and as luck would have it, there are 3 coconuts hanging in the tree right next to her. Spray them to knock them down and then put the in her basket one by one. Don't worry if some of them fall in the sea as 3 more will grow back again straight away.
Fruit Lady 2
Another fruit lady is waiting in the dock area of Delfino Plaza, outside a shop near the waterfront. She asks for bananas, which can be found at one of the fruit stalls in Bianco Sqaure and on top of a nearby building.
Fruit Lady 3
This woman is standing next to the giant Shine Gate in the dock area of Delfino Plaza. Here basket is high off the ground so you'll need to throw the fruits in.
Fruit Lady 4
The toughest fruit lady challenge, no question about it. The one wants Durians and they are the spiky ones that Mario can't carry, he can only kick them around. So it's football time.
There are a few ways to get the Durians to the basket but this seems to be the simplest. Get a Durian from the small fruit stall next to the portal to Ricco Harbor. Then gently kick it along the white wooden path aim toward the tree in the distance (this is shown in the picture on the right). If you run at the Durian and press 
just before you get to it, you will give an almighty hoof and kick it far into the distance. So when the fruit is in position, kick it toward the distant tree, then hover over and see where it landed.
Hopefully, it didn't bounce into the water (if it did, just keep trying), so from here kick the Durian through the archway and then go left to the little hill. Getting the fruit up the hill can be very tricky because of the camera angles in such a tight space. You need to place the fruit at the bottom of the hill, stand directly behind it and the spin the camera around so it is facing Mario. Now run in a dead straight line up the hill with enough speed to get the Durian up there. Once you get up the hill, kick the fruit into the basket. Now just do that two more times and you've got yourself a blue coin.
Flaming Pianta
This might sound like the name of a refreshing cocktail, but early on in the game there actually is a Pianta fellow running around the streets on fire! He'll appear after you've collected your first few shines. So, when you see him, make his day and give him a quick squirt. In return he'll give you a blue coin.
"M" Logos (x3)
There are a few of Shadow Mario's "M" tags painted on walls around Delfino Plaza. Clean them off with FLUDD to gain a blue coin for each.
- Shine Gate crate: There is a large crate just in front of the Shine Gate (on the right side). Ground-pound this crate to smash it and reveal an "M" logo and wash it off for a blue coin.
- Rooftop crate: There is a large crate on the rooftops near the portal to Gelato Beach (it's hard to miss it really. Smash the crate to reveal the "M" logo and wash it off for a blue coin.
- Arch by the red cannon: Near the red cannon that fires you over to Pinna Park there is an archway with a large tower on top of it. On one of its faces, about half way up, there is another "M" logo. Get on to the rooftops to clean it off and grab another blue coin.
Blue Birds (x2)
There are 2 blue birds flying around Delfino Plaza which turn in to blue turns when you spray them. Their locations are:
- On the roof of the crate smashing game (or very nearby)
- On the roof near the red pipe that warps you to Sirena Beach
The blue birds usually hang around with some green birds, and are very wary of Mario's presence so you'll have to squirt them from quite far away. When you squirt them they fly off for a few seconds and then land pretty much where the were before - but if you are too close they won't land again, so give them some space. If you spray them enough times, they will turn into blue coins.
Stone Pillars In The Sea
Near the tower on the east beach, where the portal to Gelato Beach is, there are some stone pillars a little way out to sea - you've probably been there before by this point. One of the further-out pillars has a double door in the bottom of it, which hides a blue coin. Use FLUDD's Turbo Nozzle (if you haven't unlocked it yet, you'll have to wait a while) to skim across the ocean and smash through the double doors and grab the coin.
Jelly Covered Hatch
On the rooftops in the north-east of Delfino Plaza, near the tower that warps you to Gelato Beach, there is a small tower with a hatch covered by yellow jelly. Use Yoshi to squirt juice on the jelly, melting it and revealing a blue coin.