How Unethical Is A Purely "Blocking" Move In Ticket To Ride?
There seems nothing unethical about such a move - opposite to your example of pretending to want to trade (which in my book is clearly unethical because it involves active lying), this move is just one of two main winning strategies:


Get more points for yourself
Limit points of the opponents


There is no lying or obfuscation - you are making it harder to win for the opponent by legal game means without any metagaming (like looking at an opponent's cards or hustling opponents into telling you some information which you shouldn't know while playing by the rules, like in your Settlers example).

Basically this move could be similar to a situation in Settlers when you put a robber on someone and also yourself - you are losing something (a turn in TTR) but in certain situations limiting the opponent more.