Writing in the 1990s and 2000s, author Albert Jack and Messianic Rabbi Richard Pustelniak, claim that the original meaning of the expression was that the ties between people who have made a blood covenant (or have shed blood together in battle) were stronger than ties formed by "the water of the womb", thus "The blood of the covenant is thicker than the water of the womb". Neither of the authors cite any sources to support their claim.
Nice, do you have a source for that so I can fix the wikipedia article? Either way it doesn't particularly matter.