On 4  occasions over a six month period our 5 year old female Bernese has shown aggression to 4 black dogs - 3 were black labradors. On the first 3 occasions we intervened before anything could happen. This most recent occasion she managed to get the male labrador by his neck and pin him down. As far as we can see there was no provocation by the other dogs and we are completely flummoxed. There are labradors that she knows and gets on with very well and we also have a chocolate labrador that she also gets on with. Until now this is completely out of character. We are planning to see a behaviourist but just wanted to know if any of you have any thoughts.
Thanks in advance

Has she had a vet check?  Any sudden change behaviour like this can (and often does) have a medical reason behind it.

Also, has she had negative experiences with black labs?  It doesn't have to be anything serious - just running up rudely can be enough.  My boy cannot be trusted off lead with any labs, particularly yellow ones, because they have run up and frightened my collie a few times in the past and they didn't actually do anything.  Just approached far too quickly and straight at her.  As a breed they can be very over-friendly and rude when it comes to greetings (usually because the owners haven't put the work in to teach them otherwise).

3 were black labradors.

I find >some< black labs can be 'pushy' with bitches, in an amorous kinda way, they can (& do) also come unstuck with the wrong bitch......in a not so amorous kinda way.

As far as we can see there was no provocation

You might be missing what body language was passing >between< them at the time & maybe from distance
.

Thanks for replying Nikita and Euro.
I will take her to the vet just in case there is anything else going on.
We did have and incident with a black dog (half lab) who was playfully barking with our bernese and she told him off in no uncertain terms as she was more interested in playing with her ball - this may have triggered the behaviour.....
We are in touch with the owner of the lab that she attacked and it sounds like he is (understandably) wary around other dogs. We have also paid his vet bills...not sure how much more we can do.

Euro - yes perhaps we did miss some signals

If cleared of any health issues, a full mot, bloods, eyesight, hearing, checking for pain etc i would want done then get onto behaviour work.

Strangely my hounds, since forever, haven't liked black Labs.   It's odd (colour) because the majority of ourl line of hounds were dark tricolour, ie mainly black.   I can't remember anything to have triggered this.   On the other hand, my Whippet can't stand Border Collies.   She's not especially keen on any dog, breed or not, but for some reason she goes nuts at the sight of a BC, and especially on TV.    There's one selling vacuums and the like, and she only has to hear the soundtrack to set her off.   Ditto with a SpecSavers ad where a farmer strips a BC instead of a sheep. 

Apart from an obvious attack, I have no idea why this happens.   But I would get your Berner checked by her vet in case there is something medical going on.

I have been told.that it is not.incommon for black dogs to be a problem for others . Reasons give are that it is more difficult for.them to read expressions .no idea of its true but I own all black dogs

It certainly is possible for dogs to dislike particular types, and black dogs are supposed to be a common one for that reason.  I can't say I've noticed a pattern in that way, although I haven't kept close track so it may be there and I've missed it.  I have several black or mostly black dogs and I haven't observed any more issues with them than my others.  I have noticed more issues with dogs like boxers/akitas/huskies (forward stance and behaviour) and pugs/bulldogs/also boxers (noisy breathing and snorting).

I do see a lot of dogs that are iffy with labs in general though, aside from my own lad.

ETA collies I understand - I have often heard of dogs having problems with them, because they are very intense, often reactive and that 'collie eye' of theirs.