Metropol

I have a flat jpeg image of black text on a white background. I want to change the colour of the text to a blueish colour, but can't seem to find a method other than the rather time consuming (and ineffectual) one of individually recolouring each letter.

It's been driving me nuts. I'd be grateful for any help, please!

Ensure that the layer named 'Background' is a real layer by double clicking on it. It'll be called Layer 0.
Use the magic wand to select the white background, then from the Select menu choose Similar. You'll have all the non-text parts selected. Hit the Delete key.
Now right click the layer in the layers list and choose Blending Options. Down the left in the dialog that appears, choose Colour Overlay. Select your blue colour and click OK.

But if it's a flat image there are no layers, not even a background one.

Have you tired using the magic wand on the background colour, going to Select => Invert and then painting over the whole lot?

It might work, but depends on how much aliasing the text has

Easiest way is to go to Image > Adjustments > Selective Color, then make sure Absolute is ticked (and not Relative), select Blacks from the Colors drop-down menu, move the Black slider down to something like -50%, move the Yellow slider down a bit too, then move the Cyan and Magenta sliders up. Then just play around till you get the shade you want.

And that would make you *what*, cdd?

Thanks for the helpful suggestions - the magic wand didn't quite work in this instance, but the colour adjustment did after a bit of tinkering.