...then going back into mothballs during the Golden Age...
Okay, I guess it's plausible, though I doubt the SL would be selling off even stripped-down hulls once the Amaris crisis began, to say nothing of the leadup to Exodus. Once the Hegemony Shopping Spree that was the 1st Succession War began I guess they could try to grab some, though taking the time to refit those things back up to combat status in the middle of *that* firestorm seems...chancy.
Honestly, I'd more expect those hulls to end up in RWRN hands. Can't remember exactly, but I'm pretty sure they did indeed grab a lot of ships like that and put them into service against the SLN.
I wonder how Black Lion Is would perform against Defenders...
Hmm...that's a really good question, actually. Armor is roughly similar, though the Black Lion has a big edge in SI. Black Lion also has the edge in raw firepower, though the Defender can more easily concentrate heavy bays due to its fire arc layout. I'm going to give the Black Lion the advantage due to the fact that its secondary guns also reach Capital Long and because of the solid missile armament, but it's only a slim margin. The Defender's raw speed lets it close the range gap quickly, and the armor levels of both ships are such that you only need to score one or two more hits than the other guy before his ship is in MUCH worse shape than yours. This would actually be a really interesting fight to game out.