In Half-Life and Black Mesa, the HECU are never referred to as Marines specifically. They were always called "The Military." In fact, Black Mesa adds more confusion with lines referencing the Army, lines referencing the Marines, and textures with USMC on them. However, the grunts look like their in the Army, and they have an Apache helicopter! So I always figured the HECU in Half-Life were a PMC, while in Black Mesa it was a joint operation. However, rather than giving them urban camo for some reason, I decided they both should get their own uniforms.
I hexed a set of models to use new textures (I called them "Barines" for hexing simplicity), while the original models (marines) are using vanilla textures. I also changed some variables in the code, but nothing too major.
The grunts randomly select the textures for the most part, but certain individuals the Black Mesa team coded to use a specific setup each time. So the cutscene grunts and the RPG grunt will always use the model type defined by the code, and I can't really do anything about that. Both grunts use the same helmets, gasmask, beret, and gloves.
Both groups gib! When you blow up a "Barine" you'll see "Barine" gibs, and when you blow a vanilla Marine vanilla Marine gibs will show. Additionally, I changed the facial system a bit. There are new faces on both sets of hgrunts, and standard hgrunts don't all wear gasmasks anymore. It is more like it was in Black Mesa's beta, where hgrunts will have different body groups. Standard hgrunts now also have the ability to smoke cigars, but there is a small bug where sometimes they'll smoke them while wearing a mask. I can't seem to find out where it is doing this in the code, but it usually is not noticeable anyways.
Also, there are optional new lens effects! These effects remove the transparency in order to reflect the world more. Think SAS on CS:GO if you want a comparison. This gives the impression that the lens are more of a "one-way" type of glass than they were before. While the original lens technically did that, the image was very dark and hardly visible due to the transparency.
Now, due to me not wanting to rerelease M3Luckycharm's textures without permission, both hgrunt sets will use the default camo upon installation. It is up to you to install a reskin in either "marines" or "barines", either will work fine.
UPDATE: Barines no longer share some texture directories. This means both sets of hgrunts can now use their own helmets, berets, gasmasks, shades, etc.