How do you figure? You still have to field a 25 man roster. None of their top 30 prospects in the OF(save for Dahl who's already up) are above AA. The OF on the roster outside of Cargo and Parra who would be starting presuming they deal Blackmon are Brandon Barnes(hitting .220/.250/.320 on the year), Ryan Raburn(.220/.304/.423) and Ben Paulsen(.258/.300/.379). Also for what it's worth, Parra is hitting .263/.274/.424. Szczur is hitting .290/.327/.439. That's not earth shatteringly good but given what they'd have left you can make a case he'd start.
And more to the point, you have to have depth even if you are rebuilding. You can't just entirely crater the team because ultimately someone has to play out there and you'd at least like someone to come see the games. Now maybe you argue they shouldn't trade Blackmon which is a fair argument but his name is reportedly out there. So, if they are talking about dealing him would you rather have Szczur who's not even arbitration eligible until 2019 or would you rather pay likely $4-5 mil for a temp fill in a la Austin Jackson(Sox are paying him $5 mil) to fill in for the loss of him next year? Also, on a per PA basis, Szczur has actually been worth more fWAR than Blackmon though that is a bit deceiving since he's been able to bat in favorable match ups more often.
You're talking about trading 2 years of control on a 2 fWAR player in Blackmon for 5 years of control on Szczur who's been worth 0.4 fWAR this year in 114 PAs this year. Blackmon's been worth 1.3 in 391 PAs. And on top of that, you're presumably getting a prospect who could be a major league player. I honestly can't imagine the Rockies are going to get more out of Blackmon than that.