You can only draw line of sight into the blocking space you target, not through the other blocking spaces the target figure (or any other figure) occupies. You thus need to target the blocking space you "see", because you can't draw line of sight through blocking spaces, only into them (if a figure or object you are targeting or interacting with is there) - or out of them if you're attacking from a blocking space to non-blocking space.
It seems that it wasn't even considered how this would work for bigger figures rather than those only taking up a single space, as this passage wasn't in the RRG for massive figures and instead added as an afterthought in the FAQ. I don't see, based on the current written rules material, that there is any clear call on the issue - or rather that you could easily argue it either way.
With the new maps just having been rotated into tournaments and with the World Championship starting on Saturday, I suspect this issue will pop up soon enough (what with lists with large massive units and lists with blast both being popular at the moment).
Edited by Cremate