Probably not. While i expect you're right that Bloodborne will chip away sales from the PS4 version of DS2, the series is considerably more relevant on PS consoles than Xbox (2:1 for both entries). Add in the much larger install base discrepancy than between their 7th gen predecessors, and the gap between the two version will likely be larger than any potentially lost sales.
You never know though. If Bloodborne is a big success (both critically and in sales), many X1 owners might become interest in buying DS2. I can't personally see that bridging the gap, but it wouldn't be the strangest thing to ever happen.