Bulldog attacks

Question
My boyfriends female American bulldog has developed some aggressive behavior around going to the dog park, barking in general, and also has developed a pattern of guarding the entrance to the dog park and being aggressive with dogs upon entering the gate.   In the last month, she got in a scuffle with another dog--a black lab mix.  Every time this dog came into the park, they would go at it and I'd have to pull her off of him.  She never draws blood, but she causes a scene and I get embarrassed--I feel like she is a "bully" at the dog park and can be really territorial there.  I feel like she thinks its her yard, and she has to approve everyone coming in.  I tried to break this behavior by making her stay near me when other dogs enter and exit, etc. and she'll stay near me and then go attack after I let her go.  She acts like shes going to be friendly, then bam!  she goes for them.  She never attacks small dogs, only black dogs.  I immediately pull her off the other dog, make sure she is okay, the other dog hasn't been bitten, etc. then I haul her out, scolding her and making her stay close by me.  I tell her NO! when shes in the fight and she doesn't stop.  When we get home I make her lay on her bed with her head down for a  half hour or so.

I thought, maybe she just doesn't get along with this dog--like people, certain personalities don't go together.  But now theres another problem with another black lab mix--they've gotten into it a few times before, but they always were okay with each other the next day.  When this other dog came into the park, I made my dog stay by me (worried about the last time they interacted).  My dog walked right up to this dog and pinned it against the fence (she was unprovoked).  Unfortunately, that is how it is with the other black dog--so now there are two black male lab mix dogs that she doesn't get a long with and I feel uncomfortable taking her to the dog park where she could potentially get into it with other dogs.  Both of these black dogs were quite dominant and slightly aggressive with her at one point...now every time she sees them she goes right for them, unprovoked.  I feel like shes racist against black dogs...my boyfriend thought that maybe she had a hard time differentiating their facial features and expressions.  Is there a history of American bulldogs having a hard time with dark colors, etc.?  She will also freak out if you have  a hood on and she cant see your face.

She has never bitten a person, is really good with children and has a great temperament otherwise--people have said that she is even quite timid for a bulldog.  She is a big girl (80 lbs) and I thought maybe living in New York city in a crowded apt might have something to do with her aggression.

I take her on a run at least two times per week--should I just stop taking her to public dog runs?  

Let me know what I can do to diffuse her and prevent her from attacking the black dogs.  

Answer
This is apparently defensive behavior she's developed after the first episode with the black dog showing aggression toward her. Obviously she's holding onto that experience. If you haven't taken her to formal obedience classes I highly recommend that you do. I would keep her away from public dog areas when there is a black dog present just to avoid any really bad situations. You have to be the one in charge here.

I'm sure in the city there must be dog behaviorists. See what one might have to say about this and if they can offer any suggestions for you. Having her spayed might make a huge different as well.