I'm assuming that yu want to make the blue more gray. Hope I got that right.
There are a couple aof quick and dirty ways to get this done. I'd duplicate a layer or 2 to make sure I don't lose the originail.. You can always hide or trash a layer later.
1. Duplicate the layer. Select Black as fill color. Use magic wand to select the blue in the new layer. Go to Select menu/Select Similar to make sure you got all of that blue selected. Go to Edit menu Fill. All the blues will be black. Make sure the layer with black is topmost Use the transparency slider on layer pallette to adjust transparency of black - giving a gray appearance.
1. Assuming you're in cmyk mode. You can use the eyedropper (if the blue is imported from a pic or something) to pick up the blue as the fill color. If it's a swatch, you can select that swatch as fill. If you open the color window and adjust the k value upward. As you see reach the color you like in in the Color window you can double click the little swatch, opening the Color Picker window. Hit OK and fill will change to that color. Select the blue in the image and Edit/Fill