After the Flood, God gave an emphatic command to Noah. This was to be all-inclusive, applying to every human descended from Noah. There was no Jewish nation, or any other nation at that time; just eight human souls preserved through the flood. At the end of all the things God said, we read:
"And I, behold, I establish my covenant with you, and with your seed
after you." Genesis 9:9 A.V.
We are all descended from that line coming from Noah, through Ham, Shem and Japheth, for "unto them were sons born after the flood" (Gen. 10:1).
The bit that speaks of never eating animal flesh with its blood still in it is crystal clear: "But flesh with the life thereof, which is the blood thereof, shall ye not eat." (9:9) Blood is not to be eaten (or drunk, for that matter, due to the principle involved.) Other parts in the Old Testament speak of pouring out the blood of the animal on to the ground, as water, and covering the blood with dust. Clearly, that would not happen with a strangled animal. Even if it was strangled to death with bits of it cut open so that blood came out, a great deal of blood would remain, pooling in some paces, but still being residual in cells all over the place, due to the heart no longer pumping blood around the circulatory system. With cutting the throat and immediately letting gravity and the still pumping heart combine, the vast bulk of arterial blood would come out quickly. (The heart would stop pumping a few seconds later as blood volume would rapidly decrease.) Cutting the throat of a live animal and hanging upside down, would also prevent the heart stopping due to other causes, which can result in the pericardium sack around the heart filling up with blood, so causing the heart to stop. The Israelites were prohibited from eating the flesh of a clean animal found dead (Leviticus 22:8). They could kill and eat lame or blind (clean) animals, but (again) only if their blood had been poured out like water on the ground, and covered with dust.
The point about the Acts prohibition for Gentile converts to Christ was that they would have no detailed knowledge of all the Mosaic laws. They would know of Gentile practices and that few had any compunction about eating blood in meat (or drinking it). Descendants of Ishmael would continue the Abrahamic way of cutting the throats of animals, but the vast majority of Noah's descendants, by the time of Christ, would hardly know, let alone understand, the detailed laws that the Jews observed. So, those Gentile converts being addressed by the Apostles simply had 'strangled animals' detailed, for they would know that such meat had blood in it.
The answer to the question, then, is that the Apostles were reinforcing the Mosaic law regarding not consuming the blood of animals. But by also saying simply "blood", every other contingency was covered, from blood-guilt in murdering a person, to drinking blood. The point was that God states the life to be in the blood in the Old Testament, and that is the reason for not consuming blood in the New Testament too.
However, the balance achieved in Acts is that keeping that aspect of the law did not bind Gentiles to keeping all of the law, necessitating becoming circumcised and avoiding 'unclean' animal food. The Apostles Peter and Paul went on to show that the law on eating unclean animals did not apply to Christians, yet the wording seems to take it as understood that blood would also be drained out of formerly unclean animals. This may raise the question of whether meat left to 'hang' with the blood still in it is allowable. But that is another question, for the individual consciences of Christians to consider. Acts 15 is an all-embracing requirement, covering all contingencies, without going into Old Testament details, which the Gentiles being addressed would hardly be aware of.
