First of all, let me say that I really hope this is something you're considering, but no pressure and I'm not going to get my hopes up.
Seems to me that many of the successful "1st b/g" scenes are not by g/g stars, but by models whose material to that point can best be described as "not hardcore," meaning erotic photography, fingers-only solo (i.e., no-penetration) masturbation, etc. Many, but by no means all, of these girls branch out into g/g scenes, but even many of those aren't even hardcore, i.e., they are simulated or "everything but" g/g erotica. Stars who actually do hardcore g/g understandably become identified as g/g stars, but I think the best transitions to b/g scenes are actually the ones who skip hardcore g/g altogether and go straight to b/g from their solo and/or 'g/g lite' material.
One example that comes to mind is Priya Rai. To my knowledge, she wasn't into g/g stuff at all until she switched to hardcore in general. At that point, she started doing g/g hardcore on occasion in addition to b/g, but the real jump was from softcore to hard. And even though not every body likes Priya, she's been extremely successful, turning out probably hundreds of scenes over the last few years, most of them for major companies. In fact, even her detractors actually complain that she won't do anal.
Briana Blair also had a very good transition to b/g, but she basically went from being a NBA cheerleader to sucking & fucking on camera overnight.
I guess the point I'm getting around to is that it seems like it may be a little easier to go straight to b/g. For longstanding g/g stars, there's such a thing as fatigue for both stars and fans. We all get used to a star's chosen niche and the transition is a little too awkward. So a star who suddenly makes the transition to b/g after years of doing hardcore g/g frequently turns out disappointing scenes. Personally, I don't care for g/g. By all accounts there's a huge market for it, but not every fan of b/g likes g/g and vice-versa, so there are undoubtedly some significant differences in terms performing in each type of scene.
Hanna Hilton's first b/g scene was hotly anticipated, but pretty awful in the end. She was clearly uncomfortable, and really didn't want to take that facial. She did a few more scenes and got a little better, but then up & quit altogether. A couple others who probably never should have taken the plunge (and who also quit immediately after) are Andie Valentino and Ginger Jolie. Either their careers were winding down & they knew it, so they figured they'd cash in and move on, or they really did want to pursue b/g performing as the next step of their careers but ended up hating it. Either way, the scenes didn't turn out great.
And for some stars, it just takes time. April O'Neil didn't have the longest g/g career before jumping into b/g, but it was long enough that a lot of people really looked forward to her 1st scene. And even though it was pretty much a dud (once again, she really didn't want to take that facial), April's scenes have gotten a lot better, and she's stuck with b/g for a couple years now.
Sunny Leone's first forays into b/g were really tepid at best, and her b/g scenes have never become that good, but she's so incredibly hot it doesn't matter.
So obviously I'm no expert or anything, but there are a couple things that seem common to the really good 1st b/g scenes:
1. The girl has to want to fuck on camera - it can't just be a career move or last ditch money-play.
2. Closely related to #1 is that she has to have some chemistry with her scene partner, and actually like facials and/or cum-play (for a really hot 1st scene, swallowing could very well be the coup de gras).
3. A lot of good firsts are exclusive to the girls' sites.
I'm still amazed Jelena Jensen made such a monumental shift to b/g successfully. Nothing against her or anything, but I never thought she'd do it in the first place, and when she did, I was afraid the different style of performing would be too much of a change for her and her fans. But I was wrong to doubt her, because she knocked it out of the park. Not surprisingly, Jelena did all three of the things I named above. Many years ago, Kelly Madison did the same thing.
One possible corollary to the above is that it probably helps if the guy is super hung and cums like a geyser. Unlike a lot of guys I run across on these types of forums, I really don't pay that much attention to the stunt cocks in the porn I watch; I don't care to laud or criticize male performers for their performances, and guys who do kinda creep me out. To me, if it's a bad scene, it's just a bad scene; I don't feel the need to bitch about the dude's performance. But when the girl actually enjoys the sex, it shows, and a monster load at the end makes even so-so scenes pretty hot.
For example, you mentioned Karlie Montana. I know a lot of guys really don't like Manuel...whatshisname, but there's no denying the dude puts out a gallon of cum, and Karlie took it all like a champ before lovingly licking his cock clean. She also gave him a rim job, let him finger her ass a little as he fucked her from behind and at one point vigorously fingered herself in a standing position in that scene.
In the end, I guess my point is a pretty obvious one. You could pretty much summarize all of this by saying that it isn't enough to actually do boy/girl, it has to be hot fuckin' porn or it really isn't worth doing.
Sorry for the long post.
