Yes, that is the correct way.
al_set_blender sets the blending mode. The default is pre-multiplied alpha which means that the color al_map_rgba_f(0.25, 0.25, 0.25, 0.25) is actually full white that has the alpha turned down to 25%. If you wanted non pre-multiplied alpha you can also see how to do that in the al_set_blender page, and in that case you would get a dark grey with 25% alpha for these same values.
Once the blending mode is correct, you want to draw your text, which you can indeed do with al_draw_text, and if you want the text to have a certain color you'd put in the correct values for the color as per your blending mode.
If you use the same color with any of the text drawing functions (like al_draw_textf you will get the same result.