I have to make a small but very important correction here. It appears that this is actually two distinct questions.
First, "If I hit the brown first and then the blue...," is it a foul? Yes, this is a foul. If a Free Ball is nominated, then the Free Ball MUST be the first ball contacted, contacting natural ball first is a foul. (It is actually also legal for a simultaneous hit of the Free Ball and the natural ball, but it is really mostly best to ignore this point since true "simultaneous hits" are a one-in-a-million rarity. If you are watching VERY closely, one or the other of the two balls will almost certainly be first contacted.)
Second question, "...pot the Brown and the Blue (in the same stroke, after having LEGALLY contacted the Free Ball Blue first)...," is it a foul? No, as I have stated the details here, this would not be a foul. If the exact situation you had in mind differs even slightly, then it may or may not be a foul, you would need to clarify the situation in order to determine legality. If a Free Ball is nominated in place of a Red and the Free Ball and one or more natural Reds fall into pockets, then all of the resulting pots result in points scored (i.e., Free Ball Yellow in place of Red and Yellow and two Reds are potted in the same stroke then three points are scored, Yellow is respotted). However, if a Free Ball is nominated in place of a Colour (such as your Free Ball Blue nominated for the Brown) and the Free Ball AND the natural Colour are both potted, then ONLY the natural ball value is scored (i.e., the Free Ball Blue is contacted first and both the Free Ball Blue and the Brown are potted then FOUR points are scored (not eight) and the Blue is respotted.)
There is an extensive thread about Free Ball and its common confusions here: viewtopic.php?f=468&t=5821