Veteran striker Kevin Lisbie overcame a bout of sickness to score in each half to help seal an eighth straight win for the Londoners, while Blackwell's side remain in the bottom four after taking two points from their last three games.
Blackwell said: "If you give goals away like we have done then you can't win games. It's a recipe for disaster.
"I just thought we had one or two youngsters who looked a bit jaded, but still we had some chances.
"Trevor Carson had one save to make in the second half and that was it. I thought they played bright little spells, but never really hurt us.
"The chances we've missed are a crime. I'm just getting frustrated now with the amount of times we're creating chances and not finishing them.
"I'm always disappointed with goals against, but we had three or four clear-cut chances.
"As a striker you can't ask for anything more than getting the ball where it was and only having to finish it off.
"When we didn't get what we wanted we started to panic and their experience showed. It wasn't enthusiasm or exuberance, just nous."