Ork wrote:20 Fighter gets +4, 30 Fighter gets +6. That's a spread of -6 AC, -2 AB, and -2 DMG from enchantments. Now look at the Blackguard. He gets +10 AC, +10 damage from divine feats. He also gets +10 saves from BG levels. What is he lacking? the 2 AB. I think for the other qualities, I'd sacrifice a bit of AB for sustainability.
Except that all of that CHA that the BG invested in can be STR, CON, or (on gear) +1 saves for the fighter. Talking about +10 saves on a BG compared to a fighter ignores the fact that the fighter could put +1 saves on every single piece of gear (which the BG would have to spend mostly on CHA bonuses), so that +10 to saves quickly looks less impressive compared with +11 saves that the fighter could pick up. It is still a good and worthwhile bonus, especially since the BG can apply it to other things as well, but it isn't so clearly superior to what a fighter can do as you're saying.
So maybe the BG can have 4 more AC when divine shield is on, but that's AC that's lower by 6 for the rest of the time. The damage increase is nice, especially since it ignores damage resistance or immunity, but all of the fighter's physical damage will ignore spell-based DR from level 25 on. These bonuses can be activated for, altogether, 19 minutes per rest with extra turning, so they aren't very limited, but they take 12 seconds to activate both and the fighter's bonuses are always on. Again, it seems to me that this isn't so clearly superior to what a fighter can do, especially when we start to talk about the fact that the BG has to have at least a base CHA of 18 and a large portion of their gear to achieve these numbers, and the fighter can invest these points into STR or CON instead to gain an advantage in another way.
The fighter will always have +2 AB compared to the BG also, and that advantage can be even greater if the fighter uses the points that the BG spent on CHA on STR instead, and is able to pick up extra Epic Prowess because of the 5 extra feats the fighter will have.
Both builds are solid, and have their strengths over the other. The main differences are RP freedom and the question of [Always on, slightly lower] vs. [Activated abilities, slightly stronger].
Maybe a good compromise build would be fighter 26 / BG 4. You pick up all of the same bonuses that you talked about before, but get an extra 3 epic fighter feats, +1 AB/damage, +3 AC, and the ever-important stoneskin/premo DR piercing.
-----------------------------
Re: fighter epic feats
This is where you have to think carefully about what feats you want that are fighter bonus feats and which are not. For example, you can take epic saves boosts only on general feat levels, not on fighter bonus feats. If you take all three of those, you will be getting a huge boost to your character and lowering the number of epic feat slots you have to think about to only 6. EWF, EWS, Epic Prowess, and Armor Skin would take up 4 of those, and now you just have to think of 2 feats that you want. If you have at least 21 CON then the decision is easy: EDR feats. At least 23 STR, and you take overwhelming critical.
There is definitely enough good stuff at epic levels to take on a fighter if you think through what you need to take on what levels. It's only when you start taking too many feats that are on the fighter bonus feat list as general feats that you'll really run out and wind up taking too many pre-epic feats as a pure fighter.