The objective of this study was to investigate the in vitro effects of blueberry juice on healthy rat aortic rings, and to explore the roles of potassium channels and of the hydrogen sulphide (H2S) pathway in mediating the effects of blueberry juice. Firstly, the antioxidant capacity of blueberry juice was compared to other popular juice drinks using the Folin–Ciocalteu and the DPPH assays. Blueberry juice had significantly higher total polyphenol content than any of the other drinks studied (p < 0.01). The effect of blueberry juice on noradrenaline-contracted aortic rings was then observed, and the juice caused significant inhibition of noradrenaline-induced contractions (p < 0.01). Voltage-gated potassium channel (Kv) blockers 4-aminopyridine (1 mM) and 3,4-diaminopyridine (1 mM), as well as the cystathionine γ-lysase (CSE) inhibitor D,L-propargylglycine (2 mM) were then utilised to elucidate the role of Kv channels and the CSE/H2S pathway. Kv channel blocker 3,4-diaminopyridine caused significant blockade at 1/100 and 1/50 dilutions of juice (p < 0.01), whilst 4-aminopyridine caused significant blockade of the 1/100 dilution of blueberry juice (p < 0.05). In addition, D,L-propargylglycine potently inhibited the effect of 1/100 and 1/50 dilutions of blueberry juice (p < 0.01). This study indicates that blueberry juice has potent vasorelaxing properties, and thus may be a useful dietary agent for the prevention and treatment of hypertension. This study also provides strong evidence that Kv channels and the CSE/H2S pathway may be responsible, at least in part, for mediating the effects of blueberry juice.