@Rowan1, it didn’t look like anyone tackled this question, so I’m happy to answer it.
Yes, Heimdall will overwrite Black Knight. In the future, I recommend using DaveCozy’s excellent (and regularly updated) thread on Status Effects - Buffs and Ailments. You can see on the Berserk buff line that it is overwritten by, among others, attack boost.
Edited to add: oh, I see it was answered, but I missed it on an initial skim. Well, I’ll leave this here since the thread reference should be valuable to you.