Oh, wait, you want details? Fine.
Without rereading the books right now, I don't remember them making a big deal of black court vamps being stronger per se, but a much bigger threat. And that is very much true- White court vamps aren't generally killers, and red court vamps don't have freakin' domination going on. Check the refresh costs. An average black court is -14, an average red is -11, and an average white is -8. The main differences? Domination instead of Addictive Saliva, Gaseous Form, and Supernatural Toughness instead of Inhuman. (Just as a quick test- try attacking something with Supernatural Toughness and Gaseous Form and nothing else. It's problematic.) White courts are down both Cloak of Shadows and Domination/Addictive Saliva. Yes, they are all equally strong- but a black court vampire is a lot harder to kill, as well being able to dominate mortals relatively easily. Red court vamps are about on par with whites in terms of a fight, but vastly better at controlling mortals and in being sneaky pains in the behind. Anytime you see refresh cost differences, you should look for a power difference.
Next, about those stats. Do you play D&D? Or at least know how it works? Six stats- strength, dexterity, constitution, intelligence, wisdom, and charisma. When you make a character, you roll a bunch of dice (Yes, yes, not everyone rolls dice, work with me a bit here) and those are the stats you get. Depending on your race, you get a +2 to some stats, and a -2 to others. Thing is, it's totally possible to wind up with a half orc with a 5 strength, a score so low that he might lose an arm wrestling contest with a housecat. This is pretty true for all systems I'm aware of- In Exalted you can make a dawn cast exalt with a single dot in strength and dexterity who loses a fistfight with a mortal, and in Shadowrun you can make a troll with a 5 body who can be outtanked by a particularly focused elf. In most games, racial bonuses indicate a natural aptitude, not outright automatic dominance. You will never be bad at it, and if you specialize in it you can be truly exceptional. Vampires in Dresden are much the same.
That vampire you're talking about has a +3 to most Might rolls without any other investment of skill. They can also probably use their "Vampire" aspect if they have a FATE point. In a skill cap 4 game, she has a decent shot at out-mighting someone who has put the maximum ranks into it. Imagine the scene- a frail looking woman in an evening gown is accosted by a beefy PC, muscle shirt stained with the sweat of his hours at the gym and his job down at the steel foundry. He grabs her roughly by the arm- and she picks him up and folds him in half, then chucks him to the side and keeps walking. That's not to say he can't possibly win this fight, but she's going to make him work for it despite putting any effort into the skill. (Gotta wonder what she did do with those skill points- Weapons perhaps? Drive?) Now picture her vampire bodyguard stepping out of the shadows- Built like the foundry worker's older brother who also plays at caber toss. Maxed ranks plus his supernatural powers puts him at around 7 Might- he's legendary without any FATE invocation, and can quickly get up into those double digit rolls that are the signal to duck and cover. It's not impossible to out-might a Dresden Files vamp, but it takes some doing, and it's never easy.
Conclusion- They are equally strong, (in terms of might and Inhuman Strength) but not equally powerful. Your players need not worry that a black court vamp is going out be excessively physically strong, but they should probably worry about the fact that small arms fire is more or less ignored due to their armor from Supernatural Toughness, and that they might be mentally dominated and forced to do the vampire's bidding.