Analysis of 62,085 hospitalized congestive heart failure patients with hemoglobin levels between 6.5-7.4 g/dL revealed that blood transfusions reduced hospital-free days by an average of 1.8 days over 28 days, suggesting overall harm from the intervention. However, machine learning analysis uncovered significant treatment heterogeneity, with potential benefits for patients receiving transfusions on hospital day 1 and those with low serum bicarbonate concentrations. This finding challenges the conventional assumption that correcting anemia universally benefits heart failure patients and highlights the complexity of transfusion decisions in this vulnerable population. The research addresses a critical knowledge gap, as anemia affects nearly all hospitalized heart failure patients yet transfusion guidelines remain unclear. The study's strength lies in its large sample size and sophisticated causal inference methods, though observational data cannot definitively establish causation. As this is a preprint awaiting peer review, these findings require validation before influencing clinical practice. The work represents an important step toward personalized transfusion strategies, potentially moving beyond one-size-fits-all hemoglobin thresholds to individualized risk-benefit assessments based on patient characteristics and timing of care.