One thing I know enhances a hard-on, lowers BP, is therapeutic to blood vessels, and also enhances muscle synthesis: nitric oxide. I have been taking Cocovia for a few months and I can notice the difference (during sex) between the days that I take it and the days that I don't, in my perception the difference is profound enough to quickly notice.
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Warning: it is expensive. I paid $220. for 6 months worth and got a discount. That is very expensive for a supplement but there aren't any other products dedicated to this use that are efficable, have low toxins (cadmium and other heavy metals, is found in most Cocoa Flavanol supplements).
Here's some research:
Cocoa flavanols: effects on vascular nitric oxide and blood pressure
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3022066/