Its very difficult to just give a build with Katana / Dark.
Its more what you want to achieve with it .
Do you want to tank/semi tank encounters .
AoE king , single target killer .
Crowd controller ?? ..
Thats the whole problem with Dark Armour set its too versatile thus easy to mess up and make it worthless.
So beter think what you want to do with the character , if for pvp then its more single target style with utility from cloak of fear (severly nerfed by the way) and the disorient
(stil not nerfed cause almost nobody takes it 
)
Sorry for the lack of information in my first post, been away from my scrapper so long that I forgot just how much could be done. Thats probably why my respec ended up messing me up so much.
I guess I'm looking for a decent single target killer as that's pretty much how I used to be, but it was so long ago that I really don't know where to start.
For Single Target you pretty much want these powers:
---------------------------------------------
Exported from Ver: 1.7.6.0 of the CoH_CoV Character Builder
---------------------------------------------
Name:
Level: 50
Archetype: Scrapper
Primary: Katana
Secondary: Dark Armor
---------------------------------------------
01) --> Sting of the Wasp==> Empty(1)
01) --> Dark Embrace==> Empty(1)
02) --> Flashing Steel==> Empty(2)
04) --> Murky Cloud==> Empty(4)
06) --> Build Up==> Empty(6)
08) --> Swift==> Empty(8)
10) --> Obsidian Shield==> Empty(10)
12) --> Combat Jumping==> Empty(12)
14) --> Super Jump==> Empty(14)
16) --> Health==> Empty(16)
18) --> The Lotus Drops==> Empty(18)
20) --> Stamina==> Empty(20)
22) --> Acrobatics==> Empty(22)
24) --> Hasten==> Empty(24)
26) --> Soaring Dragon==> Empty(26)
28) --> Cloak Of Fear==> Empty(28)
30) --> Dark Regeneration==> Empty(30)
32) --> Golden Dragonfly==> Empty(32)
35) --> Stimulant==> Empty(35)
38) --> Aid Self==> Empty(38)
41) --> Focused Accuracy==> Empty(41)
44) --> Conserve Power==> Empty(44)
47) --> Super Speed==> Empty(47)
49) --> Soul Transfer==> Empty(49)
---------------------------------------------
01) --> Sprint==> Empty(1)
01) --> Brawl==> Empty(1)
01) --> Critical Hit==> Empty(1)
02) --> Rest==> Empty(2)
---------------------------------------------
Thanks for that It'll be good to actually be able to use my hero to fight again 
You've been a big help
Get dark regen earlier, probably swap cloak of fear for oppresive gloom, it needs too many slots, but other than that a good build.
Recently my level 50 Katana/DA has been sitting collecting dust due to CoV so I decided to respec because of ED.
Big mistake.
Because I'd been away from him so long I've seriously messed him up and to stop it happening again I was wondering if anyone could suggest a good build for him. Any help would be really apreciated