Bass blockers are a good idea in some factory systems that send a full signal to the speakers. But, these filters will have no effect on the door speakers or front speaker in a BA system, because the Boston amp is already applying a high-pass filter to these outputs. The bass blocker would be trying to eliminate frequencies that are not present.
The rear deck speakers have separate wires for the tweeter and woofer. The tweeters already have a high-pass filter and the woofers have a low-pass filter. Adding bass blockers to the woofers would have about the same effect as un-plugging them, because only low frequencies are being passed from the amp, then if you block them, you essentially have little or nothing left.
If you are just adding a powered sub and not replacing the amp or other speakers, then it might be desirable to disable the 6x9 subs with bass blockers (or un-plugging them). This way you are replacing the crappy bass and not just adding better bass to it.
This still leaves one of the big problems with the BA system, which is no mid-range frequencies coming from the rear at all. There are only tweeters and subs. I think a well designed sub upgrade would also involve routing a new signal to the 6x9 speakers that includes mids, otherwise the 6x9s are wasted.