Medical research is moving toward prevention strategies during prodromal states. Postpartum blues (PPB) is often a prodromal state for postpartum depression (PPD), with severe PPB strongly associated with an elevated risk for PPD. The most common complication of childbearing, PPD has a prevalence of 13%, but there are no widespread prevention strategies, and no nutraceutical interventions have been developed. To counter the effects of the 40% increase in monoamine oxidase A (MAO-A) levels that occurs during PPB, a dietary supplement kit consisting of monoamine precursor amino acids and dietary antioxidants was created. Key ingredients (tryptophan and tyrosine) were shown not to affect their total concentration in breast milk. The aim of this open-label study was to assess whether this dietary supplement reduces vulnerability to depressed mood at postpartum day 5, the typical peak of PPB. Forty-one healthy women completed all study procedures. One group (n = 21) received the dietary supplement, composed of 2 g of tryptophan, 10 g of tyrosine, and blueberry juice with blueberry extract. The control group (n = 20) did not receive any supplement. PPB severity was quantitated by the elevation in depressed mood on a visual analog scale following the sad mood induction procedure (MIP). Following the MIP, there was a robust induction of depressed mood in the control group, but no effect in the supplement group 43.85 ± 18.98 mm vs. 0.05 ± 9.57 mm shift; effect size: 2.9; F(1,39) = 88.33, P < 0.001. This dietary supplement designed to counter functions of elevated MAO-A activity eliminates vulnerability to depressed mood during the peak of PPB.