I have no knowledge of the improvement of bioavailability Keif' Richards is talking about.
I am bit high* and I am not sure at what you are exactly confused about. But black pepper has compounds which in large enough amounts slow down/prevent metabolization of various neurotransmitters.
"MAO-A generally metabolizes
tyramine,
norepinephrine,
serotonin, and
dopamine (and other less clinically relevant chemicals). In contrast, MAO-B metabolizes dopamine and
β-phenethylamine, as well as other less clinically relevant chemicals." -wikipedia
I just tried to figure it out with just search engine, mostly crashing to paywalls, but I found out that black pepper compounds and their metabolites in have also really minor mao-a-inhibitor activity. So while that function is not as remarkable as the mao-b-inhibition of black pepper compounds, it is still increasing risk of serotonin syndrome/seizure when used along with tramadol, which also prevents reuptake of serotonin. I am pretty certain you would really have to go into deep end with consuming black pepper for this risk to be notable. AFAIK there is not even guidelines to avoid black pepper as spice on SSRI/SNRI-regimen.
Black pepper is much stronger as mao-b-inhibitor. So it increases levels of some excitatory and other neurotransmitters in blood plasma, like dopamine and phenethylamine. I tried to gather what kind of risk that would present with tramadol consumption, and apparently dopamine for example has both anti-seizure AND pro-seizure effects.
So, I am not recommending it. One capsule of some commercial black pepper/turmeric-product is probably fine, but I am not recommending it because as Keif said benefits are limited and fleeting and I don't have enough knowledge if there is realistic risk of seizure or not. Probably not, but I am not recommending it.
*I am really fucked up