L is not able to connect to B, but is B able to connect to L? You didn't say, but I will assume no.
If A is your only way to communicate between B and L then you will definitely have to log in to A at some point. And also, the data will flow through A one way or another (which might be important if A is on a slow connection).
You can establish an SSH tunnel through A in a couple of different ways to get from L to B. For example:
# From L
ssh -L 2222:B:22 -Nf A
Now add the following configuration in
~/.ssh/config on L as a convenience for connecting to B through the tunnel:
Then you can connect to B through the tunnel:
scp file B:
There are other options for setting up this tunnel, including tools to automatically establish the tunnel in the background.