1. Space colonies, especially remote ones (e.g. not Moon, but Mars or Titan), are strongly motivated to be maximally independent as soon as possible. Whoever leads them must think about the possibility that the supply ships stop coming, either for some temporary disruption (pandemics on Earth, cough, cough, or even a war back home), or permanently (something has gone seriously wrong on the blue planet). And imports from Earth are expensive, because they have to be lifted out of the Earth's gravitational well. This means that the colony will try to produce everything necessary in situ for good reasons; such a colony will not be easily brought to its knees by a blockade.
2. It is easy to block a colony physically, but they will still be able to communicate back to Earth and argue their cause in front of the rest of the humanity. This will undermine the will of Earthly powers to maintain the blockade.
3. We are now in a rare situation when only one nation (the U.S.) is approaching the capability to send ships to the rest of the Solar System. It is not unlike the situation when only the Portuguese mastered the carrack and were the only ones to sail regularly from Europe to India in the late 1400s. But this situation will not last and I would guess that by 2050, at least five or six sovereign entities will have the same capability. By 2100 for sure. And their political differences will be exploited. It is unlikely that all of them would decide to start a common punitive embargo against a rebel colony.