I understand how excited people are about him, but Blackmon should be about on par with Hakeem Nicks at this point. Both are top end talents. Nicks has injury concerns, Blackmon has suspension concerns. Both are young enough that your horizon goes beyond any reasonable projections. Both are capable of being volume guys, and have stretches where they're dominant because they're getting the looks. Both of them have uncertainty and could end up in a different situation next year that could improve or hurt their value.
If you can get top 6 value for Blackmon, you absolutely should. Top 6 is probably the high end of how anyone currently rates him, and it's hard to imagine him moving up to #5 on the list (or getting any appreciable bump in trade value) so this is a great sell high opportunity. Right now, you could almost certainly get Nicks and another strong piece for him. I'm not suggesting that Nicks is the right guy to get, just that any time you can trade one guy for a very similar guy and getting another good piece, you just added a piece. And that's how you build strong, deep dynasty teams.