Sushi is a cuisine with a high salt content, so you’re likely to get thirsty when you eat it. This will eventually affect your ability to swallow, so you will need something refreshing to relieve that. That is where the tea comes in. But not just any tea leaves will do.
One of the special characteristics of Japanese tea is that it is high in umami components. However, the higher quality the tea, the more umami components it contains, such as amino acids and glutamic acid. However, this umami does not necessarily bring out the flavor of sushi. That is because the umami in tea also contains a certain sweetness*.
*A sweet substance called theanine, which is glucose bonded to glutamic acid.
That sweet taste is not welcome when eating sushi. Sweetness has the effects of dulling taste buds to sensations of other tastes. So if you drink a cup of tea with a sweet essence, you won’t be able to discern the real taste of the sushi.
Therefore, when drinking tea with sushi, roasted tea or powder tea** with less umami is best. However, it is fine for the tea served after the meal to have more umami. When doing a taste test, you use a palate cleanser, but in this case the only options for this are water, weak black tea with low umami content, or roasted tea (now this has changed to distilled water). From this perspective, it is clear that tea with low umami content is better for really tasting sushi.
**This is a fine powder-like tea that is manufactured during the process of making green tea. The reason sushi restaurants have used it for a long time is that it is cheaper than ordinary tea, and it is not necessary to brew the leaves, so strong tea can be served immediately. It has less fragrance and umami compared with high quality green tea, making it excellent at washing away the fats and excessive fish odors.
Umami contains a moderate amount of sweet components, so it has the effect of making you unable to distinguish the special tastes of sushi. Therefore, a roasted or powder tea with little umami goes well with sushi.