Whilst they couldn't possibly have known about our medical advancements etc, the way in which it is written doesn't denote the medium by which blood is ingested, transfused or any other means. It merely says to ABSTAIN!
Another way to look at Acts 15:29 is simply on grammatical grounds. What the JW's have done in the past is ungrammatical.
For example: I can easily state what it means to "abstain from fornication" as a simple finite negative:
"Do not fornicate."
The reason I can do this is because fornication is the name of a finite act and therefore has a verb form, unlike "blood" and "things strangled" and "things sacrificed to idols" which are the names of physical objects.
Can you state what it means to "abstain from blood" as a simple finite negative?
"Do not ________"
No matter how hard you try, you will be forced to insert the name of a finite act in the blank above. The reason for this is simple:
There is no such thing as abstinence from a physical object.
You can only abstain from acts done in connection with objects.
However the only acts you may legitimately insert in this blank are those which are supported by the immediate context. (e.g. "Eating" or possibly "Tasting" --Compare Phillips, Moffat, TEV)
When the JW's play the grammatical dirty trick of invoking the incomplete predicate, "abstain from blood" as an independant construction they have simply taken advantage of the general lack of education among their members.