A 50-year-old man on ICU has severe septic shock requiring high-dose noradrenaline. Despite optimisation, his MAP remains 55 mmHg. He has received hydrocortisone 200 mg/day for 48 hours. His methylene blue serum level returns elevated nitric oxide metabolites. The team considers IV methylene blue. What is the mechanism of action of methylene blue in refractory vasodilatory shock?