I hadn't heard of that expression before, so I looked a little at the corresponding Wikipedia page [0] and the sources that talks about this quote [1 and 2]. I also looked at Quora question about the quote [3].
From what I understand, there is no way one with any certainty can say that: "the blood of the covenant is thicker than the water of the womb” => "blood is thicker than water".
There doesn't seem to be a direct "the blood of the covenant . . ." quote either.
My own thought: I think it is possible that the meaning of both quotes can be true at the same time:
a) If you make a blood covenant with someone => Strong bond, and maybe stronger than with family members.
b) If something serious happens in your life, family bonds tend to be stronger than that of a friend, at least if you don't have some type of covenant (in a broad sense) with that friend.
[0] https://en.wikipedia.org/wiki/Blood_is_thicker_than_water
[1] https://archive.org/details/shaggydogsblacks00jack/page/94/m...
[2] http://www.bac2torah.com/covenant-Print.htm#SectionII
[3] https://www.quora.com/Where-did-the-phrase-The-blood-of-the-...
Somewhat off-tangent here, but this is a frequent misunderstanding of this particular quote. The actual quote is: “the blood of the covenant is thicker than the water of the womb” and it means the exact opposite of what we now use it for. The intended meaning is that relationships entered into as adults are much stronger and deeper than those we are forced into through accident of birth simply because they are intentional. While I hope my children will be there for me in old age, I know that I will be there for my wife and closest friends even though we are not bound by any particular genetic imperative.