Being a loud, bullying, manipulative liar isn't a prerequisite for being a leader. If it were then only people like Blake could be leaders. Usually, at my work, leaders like that are detested and hated and no one respects them. People who lead like Avon, who know what they're doing, who don't throw their weight around constantly, who don't insist their way is the only way, they are the ones who are loved as leaders, even if they are cold emotionally. They aren't flashy leaders but they are competent ones. They are rational, logical and responsible who are able to think big picture and long term. They are able to understand complexities and know how to problem solve by fixing the problems. This is Avon. Blake? Not a hope. He's the kind of amateur who comes up with emotional, knee-jerk bandaid solutions that usually end up destroying everything and solve nothing. But they look really good and flashy solutions on the surface and that is typical Blake.
Avon wanted to be leader and he wanted to take leadership from Blake, that was very clearly specified several times. Avon never hid that. Of the two, Blake had the advantage as leader, he was more charismatic, forceful and was much more of a bully. That doesn't make Blake a good leader nor does it make Avon not a leader, just different types of leaders, with one people are more inclined to follow. Though they will follow both types, just as it works in the real world. To assume that just because Blake was a leader then Avon couldn't also be a leader too is a very limited and narrow way of thinking.
Even when Tarrant came onboard and tried to take leadership from Avon, he never succeeded. Watch their interaction carefully. From the beginning, even when Tarrant was trying to give orders, he always recognized that when there was anything important to be done, when there was real danger or if there was a policy decision to be made, he always asked Avon's permission.
Avon kept a careful eye on Tarrant's performance. If Tarrant looked like he couldn't handle things Avon immediately stepped in and took over and led the others in what needed to be done. Avon left the dreary day-to-day operation of the ship to Tarrant.
What does this sound like? This sounds like Avon was the exec and Tarrant was the junior officer who took care of less important and boring tasks. Tarrant didn't realize this at first. He thought that because Avon let him shoot his mouth off that he was the leader, but he learned soon enough and it became very clear Avon was always and only the leader.
And Avon never followed Blake. Avon never acknowledged Blake as leader. He was only on the ship for his own personal agenda. Any actions he took on the ship was because he was a team player or Blake bullied him into it. Avon never followed despite himself. He was a failure as a follower because Avon was never a follower, he was a leader in conflict with another leader who was a bully. The only time Blake ever gave Avon a real choice on a mission, Avon categorically refused to go, and he didn't. Blake learned that the hard way.
Avon needed time alone, but he was never a loner. Avon constantly sought out companionship, something we never see Blake doing. Avon played board games with the crew, he shared meals with them and had chats, he went with Vila to have some fun. Even when he was working and didn't need to be with everyone else, Avon hung around the others. That's a very strange definition of a loner. In comparison, Blake never did any of those things with the crew. The only interaction he had with them was by necessity, it was never social. So who was really antisocial according to canon? It was Blake, not Avon.
From the beginning to the end, Avon was usually right about everything. He had a far greater and deeper understanding about people and how society worked. Blake was completely clueless. Avon knew it was all about money and power, that's how the world runs. Even in our society that is true. Vila was right that Blake strutted around arrogantly thinking he could handle everything and he knew all about handling the Terra Nostra, when in reality he didn't understand a thing about the lower classes or the underworld. Avon knew though, he knew Blake's silly plan would never work. Like most of Blake's plans, it was an ill-conceived failure that this time didn't even have the benefit of luck.
Blake understand what it was to be human? If he didn't understand those things which should be common sense, he doesn't have a hope of understanding what it means to be human. The only one who did was Avon.