Isn't trading always blocked? Whenever you would allow someone to build a complete Color it's better not to allow it, and whenever you also would it's only advantageous if your color is stronger than the other thus the other party should never agree.
Any other exchange becomes pointless as you won't be completing a color anyways.
You seem to reason in terms of trying at all costs to block the other players, not to maximize your gain. I agree the ultimate goal is to do better than all the others, but the tactics to get there may involve temporarily favoring other players in win-win exchanges to grow quicker, or to block a third player already far ahead, etc.
In the second case, the stronger color can be offset by giving the weaker color more cash / other properties.