@StarJo - You could use one of two methods. You can either use the Duotone menu option, or you can make a blue blended layer.
To make a duotone from the menu, first grayscale the image. Then go to the Image menu and choose Mode and Duotone. Click on black and make it any shade of blue you wish. Then, click on the duotone curve, which is the box beside the color square. Move the line around until you get the desired effect.
To make a blue blended layer, first convert the image to grayscale. Then, convert it to CMYK again. This eliminates the colors, but it will allow you to add color. Make a new layer and make sure it is above the photo’s layer. Fill this new layer with blue. Then, go to the Layers palette under default effect and choose Overlay. You can pull the Opacity up or down to adjust the contrast.