The prohibition against blood was not part of the Covenant of Moses. It actually originated with God’s covenant with Noah:
Noah long preceded Abraham (the father of the Jews). Noah was the father of many nations, including Gentile ones.
While the Council of Jerusalem was comfortable putting aside Mosaic laws, it was not so comfortable doing so with regard to a covenant that was much older and more universal. Thus, they retained this particular dietary law. We do not know their exact reasons, but we can assume they felt it was the right decision at the time.
But Jesus also subjected ALL laws to the authority of the Church, with full divine enforcement, without any exceptions whatsoever:
Binding and loosing were rabbinical terms which means to impose or remove an obligation. Over time, the Church has exercised both the authority to bind and loose with regard to dietary rules - for example, requirements for fasting and abstinence. At the Council of Jerusalem the Apostles felt it wise to loose many Mosaic laws, but continue to bind one of the laws of Noah. This decision is not binding upon all future generations of Catholics - the Church has full authority to change such rules at any time (the Church can always change rules, but never doctrine - nobody ever claimed that the rule against blood was an absolute moral standard).
Likewise, the Church has the authority to determine customs of dress - both for the laity as well as clergy and members of Religious Orders. Such rules can change as often as the Church deems appropriate - although nobody ever accused the Church of changing such outward customs too frequently - usually centuries intervene between changes.
No Council or Pope can ever change doctrine - and none have ever done so. But dietary rules and clothing fashion are not doctrine, and they have changed many times.