Hello there. The fishy taste you are experiencing is likely due to the oxidation of the olive oil or specific fatty acid compounds interacting during the high-speed blending process. When olive oil is whipped aggressively into an emulsion, it can release certain volatile compounds that mimic a fishy or metallic flavor, especially if the oil is slightly older or exposed to light. To avoid this next time, I recommend using a neutral oil like canola or grapeseed, or blending a lighter olive oil with a neutral oil rather than using standard olive oil by itself.
I hope this explanation helps you achieve a better result for your next batch of mayonnaise.