Help with designing a multi relational schema / domain model
It might help if you also had some table descriptions in your question, but here goes...


  A network has many blocks.


You could do this very simply with a network_id in each block. That way there is no restrictions on how many blocks can reference a network, but you ensure that each block is only in one network.


  Each block belongs to a single client. A client may have many blocks in one network, and can also have many blocks in many networks. 


Having a client_id in your block would ensure that each block can only reference one client, but a single client could be referenced by many blocks (and it won't matter which network they're in). So maybe your block table needs to look something like this:


block
-----
  ID
  client_id (FK to client.id)
  network_id (FK to network.id)
  [other fields...]



  I also then need to be able To browse a client and query all his blocks which he has


The SQL for that could look something like:

select client.name, client.id, block.id
from block
inner join client
on client.id = block.client_id
where client.id = $SOME_PARAMETER_FOR_CLIENT_ID