Other than adopting the changes to Single Shot, Semi-Auto Burst, and Full Auto Fir from Black Crusade, I suggest against adopting anything else. It really is better to leave each game system (or each rule set) self-contained. But, if you do adopt melee changes from Black Crusade, then adopt all of them. But you'll have to read them several times and do the comparisons with Deathwatch, and I recommend you do some testing on your own before dropping it into your game.
Melee combats will go one of two ways: they will be over far faster than you'd like, or they will drag on and on and on…and it's all left to the roll of dice, which you cannot control.
Yeah, the idea would be to go all or nothing with them. However, the changes to swift/lightning attack seem like they can be problematic when fighting hordes (a good roll makes for a ton of hits on the horde), and against single, big enemies, make combat far more swingy (a character with the lightning attack talent will take 3 attacks in DW, which yields more consistently average results than the BC method, which makes it a far more "all or nothing" affair, even if, on the long run, the number of hits would be comparable, statistically speaking).