This is the most curious little gesture. Avon has just delivered the worst sort of insult to Blake during his worst moment of weakness, and the way Blake carefully puts his hand on his shoulder would infer he's about to beat him to a pulp but something else is going on there, something quite odd. Particularly as this is around the time Avon has both announced his intention to have Liberator but also effectively committed himself to Blake (can't have both, Avon!)
I wish I could have been a fly on the wall during script read throughs so I knew just what they were all thinking. I can watch it ten times and have a different theory each time.