How do Roman Catholics reconcile Acts 15:20 with transubstantiation? I'm curious how a Roman Catholic Christian would defend the practice of transubstantiation in light of Acts 15:20 which states that Christians are "to abstain...from blood" (τοῦ ἀπέχεσθαι ἀπό...τοῦ αἵματος ).


  But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.


(Not that I need to post this disclaimer, but I do believe in transubstantiation.)
Transubstantiation for a Catholic is from the philosophic concept of substance and accidents which could perhaps be phrased reality and appearance.  The substance is the reality of the thing, in this case bread or wine and the accidents is what it appears to be.  During the consecration, each element (bread and wine) are changed in substance while the accidents remain the same so, for example, the wine is changed into the blood of our Lord really and truly in the sacrament while still looking, smelling and tasting like wine.

The question equates the changed wine with the substance &amp; accidents of blood which is not the case here.  The blood referred to here is both blood in accidents and substance.  While the Greek αἷμα can refer to literal blood or those things that refer to blood such as the blood of grapes (eSword G129, Thayers &amp; Strong's), one must take this commentary reference into account:


  20. (in part)... The four classes (also v28; 21:25) seem to be four of the things proscribed by Lev 17-18 for aliens residing in Israel: meat offered to idols, the eating of blood and of strangled animals (not ritually slaughtered) and intercourse with close kin (see Lev 17:8-9, 10-12, 15 [Exod 22:31]; 18:6-18). 21. Supporting the four clauses (gar) is their recognition factor, based on the universal Torah instruction.
  The New Jerome Biblical Commentary, pg 752


This reference to Leviticus is not a sacramental but a literal reference, hence requiring the blood (αἷμα) referred to in the passage to be meant literally as blood and not in some other way, hence there is no contradiction as it might appear on the surface.

This is a good article from the old Catholic Encyclopedia that discusses transubstantiation http://www.newadvent.org/cathen/05573a.htm#section3.