Resurrecting this thread in the hope that the new influx of experienced trainers may come up with a solution.
Bear in mind that this has to be dealt with out on walks as it's strange Black Labs that he reacts to, once he knows them, there isn't generally a problem.
He has been pounced on, bounced on and generally abused by out of control Black Labs, a huge amount of which live near here

Oh bless him! he is a character your Alfie lol.
My friends dog was like that (also a goldie),he and my Harvey (black lab) were bestest mates in puppyhood but as he grew he hated black labs!!
Did Alfie have a run-in with one when he was little or get scared by one at any point?
I know these are very silly ideas but.....
Can you try and ask a few people with black dogs to meet you out on walks,make sure that all the dogs involved are sound.Just exposing him to them as often as you can.Any close friends you have with black dogs,ask them round to your garden and go for walks together,just try and de-sensitise him from the colour black.
Such a shame you are so far away,as id happily meet you on walks with my girls,but youre just too far!!

When I first got him he was the most submissive dog ever, I don't think he had been socialized properly, he had manners most of the time, but he has just been abused by so many Black Labs that he is always on the defense now until he knows them

Sometimes i just think our dogs have to have their little 'odd' things.The good thing is that he is happy with them when he knows them.
Maybe when you see an owner of a black lab when you are out,just ask if they would mind walking alongside you for a bit.
My friends dog never really stopped hating other black labs,hes old and poorly now so doesnt apply now really,but she just realised that was how he was going to be regardless of what she did.