Blue is an essential color for the artist's palette, ranging from ultramarine to prussian to indigo. Some artists (most notably, Picasso) even have a "blue period" in which they paint using mostly blues accented with other colors. Here are a few examples of some paintings which emphasize the mystery and beauty of blue.
Blau, Vasily Kandinsky
Blue Water Lillies, Claude Monet
Lady in Blue, Camille Corot
Starry Night, Vincent Van Gogh
The Old Guitarist, Pablo Picasso (Blue Period)
Still Life, Patrick Caulfield
Dance (I), Henri Matisse
Abstraction Blue, Georgia O'Keeffe
Red and Blue, Ellsworth Kelly
Blue Dancers, Edgar Degas (my favorite)
Blau, Vasily Kandinsky
Blue Water Lillies, Claude Monet
Lady in Blue, Camille Corot
Starry Night, Vincent Van Gogh
The Old Guitarist, Pablo Picasso (Blue Period)
Still Life, Patrick Caulfield
Dance (I), Henri Matisse
Abstraction Blue, Georgia O'Keeffe
Red and Blue, Ellsworth Kelly
Blue Dancers, Edgar Degas (my favorite)