In Gimp can I change all black pixels in a picture to blue pixels?

Another way is to use the Color Exchange function.

Which is located under Colors > Map > Color Exchange and then either select the color from the preview picture using the color picker tool or just define the color to replace and the new color manually.

Here is the documentation page for that function.

Replacing a colour can be done like this:

1. Activate the "Select by colour" tool:

   

   • In the tool options, choose a threshold (0 if you want it to be precisely rgb(0,0,0))

2. Click on a black pixel

3. Select your new colour and drag it onto the image