My current blackguard-in-progress build is planned to medium-or-heavy armor and use a greatsowrd for the reliably high base damage, which should get even better with sneak attack and greatsword critical. (Though if others have thoughts on other weapons, I'll hear them.)

Right now I'm bouncing between either a pure fighter who would have to wait until level 10 to be able to get the required 5 ranks in hide while possibly taking the able learner feat to learn hide (and other useful thief skills like move silently, open lock, and disable device on the cheap) vs. an evil fighter with at least one level of thief to make hide a class skill and get the requisite 5 hide ranks immediately.

The F/T will be able to pick up blackguard much faster by as earlier as character level 7 (6 levels of fighter for the BAB 6, 1 level of thief for 5 ranks in hide), and will have sneak attack from the get-go, though if I understand correctly, it will also suffer from cross-class XP penalties from having more than 2 classes.

The potential XP penalty is about the only thing stopping me from going F/T.

(The character is human to max out on feats and skill points early to get greatsword focus, greatsword specialization, greatsword critical, power attack, cleave, etc. as quickly as possible. No other race in the game seems to offer enough in the way of better stats to justify trading off the extra feat and skill points as far as I can tell.)

I don't think prestige classes give you xp penalties. So you should be safe with a F/T.

I'd start out as a rogue, giving you a lot more skill points to start with, that way you can open locks till you get the rogue NPC.

Can you sneak attack with greatswords? I'm not really familiar with 3rd edition, but in 2nd you could only sneak attack with thief weapons.

Correct, a PrC doesn't contribute to XP penalty.

Just go Rog/Blk. By the time you pick up your Blk level at 8, you'll already have evasion and 4d6 sneak attack, along with every conceivable skill in the book. All at a cost of 28 hitpoints and weapon spec (which typically doesn't shine so much over 4-24 points of sneak attack ;) )

I gave Neeshka a greatsword for one fight just to try this out, and it seems she can't do sneak attacks with that weapon. Same goes for katanas :(
She can sneak attack with bows, though.

One theoretical question: does anyone know what happens to a blackguard if his alignment shidts to good? Does he become fallen, or something...