I can find no reputable sources substantiating the phrase "the blood of the covenant is thicker than the water of the womb" as the root of "blood is thicker than water".
Jbeldock mentioned that references the Troy Book (c. 1420), but the reproduction I found doesn't seem to mention anything remotely like "blood is thicker than water". In fact, "blood" and "water" never even appear within four lines of each other (maybe more, but that's the closest I looked).
Starring The Tankard clan, Thicker Than Water continues to follow Ben and Jewel Tankard with their blended family as they live, love and laugh through the perils and pleasures of life.
"Blood of the covenant is thicker than water of the womb."basically it is stating that an action, decision or promise made by choice is thicker than that of one forced by biology.
Thicker Than Water
|Year of Release||1999 (United States)|
|Cat # / Distributor||3019-3 - Palm Pictures|
|Format Details||VHS (Slipcase)|
|Running Time||N/A (NTSC)|
|Original Title / Year||Thicker Than Water (1999)|
|Submitted By||112 Video on 08/21/13|
|Keywords||2-Shot, Crime, Hip-Hop, Rappers|