S4, I've been diagnosed w/ both BPD and BP. I can tell the difference b/w the two very well. The BP mood changes, like you occur very gradually, until it hits a point of no return when either I'm in a full blown mania or in a bleak depression. It can take me weeks or months to break out of.

However, w/ the BPD, the cycles usually happen very very quickly, and I can pin point a specific event that caused it. For me I usually lash out w/ severe anger that can lead to aggression. But, when I'm in a BP caused depression, some events that trigger the BPD will add to and aggravate the BP.

From personal experience the BPD can really piggyback on the BP.

I hope all that made sense.