In addition to the great suggestions above... which I must say again, Coleus!
Veronica, Lamium, Commelina, Gibasis geniculata (Tahitian bridal veil)...
In the shade, flowers that aren't really light-colored, preferably white, can be disappointingly invisible if you'll be viewing them from a sunny spot. If you're in the shade too, then you'll be pleased with what you see, at least that's what happens to me.
Petunias probably need more sun than what you described.
Wax Begonias will bloom constantly but don't have a blue variety. Most Begonias would be happy in your 'barrow, the cane ones can get tall, and in that genus are an amazing selection of incredibly beautiful leaves, in addition to the wide range of flower shapes (no blue to be found, but plenty of white.)
There are so many beautiful leaves for shade out there, that's how I decorate the shade every year. If you'd like to see pics and more of the plants, that's the web address at the bottom of my entry. My blog isn't one of those that gets added-to, at least not yet. Just an article about beautiful leaves.
About Impatiens, one of the saddest gardening things ever has happened to them, the champ of flowers in the shade. A disease called downy mildew (not the same thing as powdery mildew) is killing them globally. It's spread when the wind blows, and AFAIK, there is no cure once infected.
If you want to go wild and jungly, an elephant ear would love to be the star of the show, especially if the 'barrow doesn't have any holes or cracks for drainage. EE will gladly drink it dry probably daily once it gets going and the heat is on. The have them in bins at BBS's this time of year, look like weird potatoes. The bigger the spud, the bigger the leaves you'll get - generally.