How do kosher food preparation requirements compare with what is known by modern science to make food safe for eating?

While some of the kosher rules are food-safety rules developed by the Israelite priests (or given by God), the prohibition against mixing meat and milk is because of an ancient ritual that involved cooking the meat of a slaughtered animal in its mother's milk, a religious practice forbidden in Exodus 34:26 "... You shall not boil a kid in its mother's milk", part of a passage sometimes known as the ritual decalogue that prescribes the destruction of artifacts of other religions, forbids the creation of idols, and lays out other religious responsibilities.

The laws regarding separation of meat and milk come from the concern that a person might accidentally break this law and boil a kid in its mother's milk if they purchase both meat and milk and prepare them together, or if a residue of milk is left on cooking utensils from a previous dish, which then gets into a dish involving the meat of a slaughtered kid.

Interesting question and I'll try to give an answer to the questions that you state explicitly.

Blood is edible and is eaten in loads of countries. Black sausage comes to mind. However, blood tends to coagulate rather fast and meat needs a fair amount of time to mature. I'm guessing that leaving blood in the meat will give the meat dark spots that are not appealing, therefore the prohibition.

Milk and meat are eaten together at least in Spain. The only reason I can imagine for not mixing them is historical. Originally the Jewish tribes lived in a desert. A hot climate and food... don't go well together. Maybe milk turned sour fast, or it turned into yoghurt, and maybe that was feared...

All this is just guessing, so anybody that finds a valid reference is welcome to post here.

The great Jewish thinker Maimonides argued that (basically) all the laws of forbidden foods exist for health/scientific/safety reasons, and he laid out the connections. In Guide for the Perplexed 3:48 he writes the following:

I maintain that the food which is forbidden by the Law is unwholesome. There is nothing among the forbidden kinds of food whose injurious character is doubted, except pork (Lev. xi. 7), and fat (ibid. vii. 23). But also in these cases the doubt is not justified. For pork contains more moisture than necessary [for human food], and too much of superfluous matter. The principal reason why the Law forbids swine's flesh is to be found in the circumstance that its habits and its food are very dirty and loathsome. It has already been pointed out how emphatically the Law enjoins the removal of the sight of loathsome objects, even in the field and in the camp; how much more objectionable is such a sight in towns. But if it were allowed to eat swine's flesh, the streets and houses would be more dirty than any cesspool, as may be seen at present in the country of the Franks. A saying of our Sages declares: "The mouth of a swine is as dirty as dung itself" (B. T. Ber. 25a).

The fat of the intestines makes us full, interrupts our digestion, and produces cold and thick blood; it is more fit for fuel [than for human food].

Blood (Lev. xvii. 12), and nebelah, i.e., the flesh of an animal that died of itself (Deut. xiv. 21), are indigestible, and injurious as food; Trefah, an animal in a diseased state (Exod. xxii. 30), is on the way of becoming a nebelah.

The characteristics given in the Law (Lev. xi., and Deut. xiv.) of the permitted animals, viz., chewing the cud and divided hoofs for cattle, and fins and scales for fish, are in themselves neither the cause of the permission when they are present, nor of the prohibition when they are absent; but merely signs by which the recommended species of animals can be discerned from those that are forbidden.

The reason why the sinew that shrank is prohibited is stated in the Law (Gen. xxxii. 33).

It is prohibited to cut off a limb of a living animal and eat it, because such act would produce cruelty, and develop it: besides, the heathen kings used to do it: it was also a kind of idolatrous worship to cut off a certain limb of a living animal and to eat it.

Meat boiled in milk is undoubtedly gross food, and makes overfull; but I think that most probably it is also prohibited because it is somehow connected with idolatry, forming perhaps part of the service, or being used on some festival of the heathen. I find a support for this view in the circumstance that the Law mentions the prohibition twice after the commandment given concerning the festivals "Three times in the year all thy males shall appear before the Lord God" (Exod. xxiii. 17, and xxxiv. 73), as if to say, "When you come before me on your festivals, do not seethe your food in the manner as the heathen used to do." This I consider as the best reason for the prohibition: but as far as I have seen the books on Sabean rites, nothing is mentioned of this custom.

The commandment concerning the killing of animals is necessary, because the natural food of man consists of vegetables and of the flesh of animals: the best meat is that of animals permitted to be used as food. No doctor has any doubts about this. Since, therefore, the desire of procuring good food necessitates the slaying of animals, the Law enjoins that the death of the animal should be the easiest. It is not allowed to torment the animal by cutting the throat in a clumsy manner, by poleaxing, or by cutting off a limb whilst the animal is alive. (Friedlander translation)

If any of this seems scientifically questionable, keep in mind that he was writing this with the knowledge of 12th Century science.