But didnt Blade only originally help Cruz because he thought he was a Needless? Since Testaments only go after Needless. One of the first things he asked Cruz was what his fragment was. After he got in front of that testament he was marked as a hostile so he had to fight, his image sent to Arclight, showed he matched, he became his target.
Just seems like Blade was in the wrong place at the wrong time. Plus he owed Cruz one for saving his life. although Blade was pretty nice to Cruz before Eve made her appearance...
http://www.onemanga.com/Needless/2/12/