I would avoid both Blatche and Darko. Both are chronic underachievers.
Calling Blatche an underachiever is not really fair he has posted 9pts 6reb for his career. Being the 50th pick in the draft I say thats decent returns. Now if you want to call him underachieving due to his contract size sure, but that isnt Blatche's fault. Wizards decided to pay an mentally unstable kid with OK production the money.
In regards to Darko he should have never been the 2nd pick in the draft. European centers never work out (The great Arvydas Arbonis is the exception). Dumars doomed this poor kids career, he was never meant to be anything else then a servicable back up. his best season came in 2010 with 8pts and 5rebs per game. So I guess he is an undeachiever because he was overrated by a GM not due to talent.
If I were to choose, I would rather go after Robin Lopez before these two. Having said that I would go with Darko over Blatche. Soft athletes are my biggest pet peeve. Mark Blount being the most hated Celtic of all time in my book.