If Blackmon is on the board, you absolutely pull the trigger. Move back up into the first and snag Weedon, call it a day.
What would you have to give up to get from #42 to #21 or #20? Sure, if Cleveland passes on Weeden at #22 then you could trade up to the high 20's or even the top of round 2, but they're not letting Weeden get past #22 (especially with their latest "ya, we'll take him if he's there in the 3rd" smokescreen).