the hypoxia and the "blood sample appears brown" should point you towards methemoglobinemia.
if you want a fun video from "9-1-1" that actually explains a lot of it: https://www.youtube.com/watch?v=LFmlRzQ-0RA
"Spontaneously formed methemoglobin is normally reduced (regenerating normal hemoglobin) by protective enzyme systems, e.g., NADH methemoglobin reductase (cytochrome-b5 reductase), NADPH methemoglobin reductase, and the ascorbic acid and glutathione enzyme systems. Disruptions with these enzyme systems lead to methemoglobinemia."
I was wondering why a baby could get methemoglobinemia. Some answers in the link.
The congenital type is due to a deficiency of the enzyme NADH-cytochrome b5 reductase, which impairs the ability of hemoglobin to bind oxygen, leading to poor oxygenation of body tissues. Cyanosis and hypoxia result. Source: https://www.visualdx.com/visualdx/diagnosis/methemoglobinemia?moduleId=101&diagnosisId=56035