Slitherine

Playing as Carthage I'm at war with Syracuse for a few years now. Bruttani (at the tip of the Italian boot) is my client state and as such also at war with Syracuse.
Now if I look at the trade panel I see that Syracuse is happily trading through Bruttani with Tarentum and Rome. Is that supposed to work like this?
Looks a bit strange to me that I can block trade over seas but not over land.

Yes that's expected. Contrary to harbor blockade, you can't really set up guards all through the border, that's not modern times.