Jack Simth said:
Might want to swap out a level of Rogue for a level of Monk - pulls a d6 off the sneak attack, but adds Wis to AC as well.
If you're slapping in Monk, take Kung Fu Genius from Dragon # 319. Preq. is Int 13, and you can't take it after taking a level of monk. Changes your stat for purposes of Monk abilities from Wis to Int, which you'll probably want for more skills anyway.
In the event that the PC is in Heavy Fortification armor, or is otherwise immune to crits and sneak attack, take Precise Strike, from Dragon # 310. For any attack that would be a sneak attack, you can exchange sneak attack dice for a +2/die circumstance bonus to hit. This, of course, can then be used to crank up your damage via Power Attack, assuming, of course, that you have a ghost touch weapon around. (The build above has Power Attack already assumed, since Blackguard requires it, IIRC).
Craven, from Champions of Ruin, is another good feat. In exchange for a -2 Will save vs. Fear effects, you do +(character level) on sneak attacks. PCs really don't do much in the way of Fear effects.
Since you have arcane spells from Assassin, Arcane Strike from CW might be useful.
In general, it might be a good idea to take just enough Rogue and Blackguard to hit 20th level with Assassin-10, and then take Epic Assassin levels for the bonus feats.
Brad