Ihenacho left Washington's season-opening loss to Miami last Sunday with a fractured and dislocated left wrist. Coach Jay Gruden said Monday that Ihenacho would undergo surgery.
The fourth-year pro started 14 games for Denver in 2013. Sunday was his first start since joining the Redskins last season.
Blackmon joins Washington after Monday's announcement that corner Chris Culliver would serve a one-game suspension for violating the NFL's personal conduct policy.
Blackmon joins his fourth NFL team. He played in 23 games with Jacksonville during the past two seasons.
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