No you can't cycle through Blackwall. The other two options are Woolwich Ferry but that is the London side of Shooters Hill (unless you climb it just for the hell of it and drop back down into Woolwich) and the Dartford Crossing but as Ian has pointed out the land rover at the crossing only takes 2 bikes at a time.
They can take 3 bikes without a problem.
Going via Dartford (as well as offering the delights of Shooters Hill) goes provide a certain
frisson by forcing the bods at the crossing to actually go out of their way and give a free service to cyclists. And therefore I think we should use the service as much as possible. But that's my personal opinion.
Whilst there could be a delay in getting the first run across, at least then they should come straight back for the next lot, so perhaps another 10-15 minutes. We could of course run 2 strands, one for those who want to go north of the river and the other south of the river. Just a thought.