Start with a good backing track in a good blues key... I like E... Then you can work off of that. Since you're not going to have vocals and its a blues song... concentrate on high points and low points of the song... When I play blues (mostly all that I play) I try to tell a story or show a timeline of emotion as the song goes by... A good example is Texas Flood by Stevie Ray Vaughan. I know it has vocals... but listen to how he has some slow parts and then he has some pretty insane runs. Also try to give a listen to Down, Down, Down by Joe Satriani... its an instrumental and obviously not as bluesy but you get the idea what I'm trying to say. If that seems hard just get a good backing track going and jam over top of it a lot and if you get a good riff or lick going try to remember it. Sometimes you can write a complete song, even instrumentals, based around one 5 second lick.