I've said it before and I'll say it again: rice is by far the single most important element of sushi. Procuring and preparing good quality seafood doesn't matter much if the rice is not good. Sushi chefs are judged first and foremost on the quality, attributes, and taste of their rice.
One element that greatly affects the taste of sushi rice is the type of vinegar used to season it. Chefs use either rice vinegar (kome-zu), red vinegar (akazu), or a blend of the two when seasoning sushi rice.
Akazu (also called kasuzu - 粕酢) vinegar is made using sake lees (sake kasu - 酒粕) which is the yeast slurry left over from sake production. It is considered by many to be the vinegar of choice for traditional Edomae-zushi. Rice vinegar was expensive during the Edo period so aka-zu was used as a cheaper, delicious alternative.
Aka-zu is typically aged for three to five years, so ironically these days it is more expensive to produce than rice vinegar. Its reddish-black color (see image below) darkens the sushi-meshi - the more akazu is used, the darker the rice will appear.
Akazu is a critical component of Edomae-zushi. It brings a deep, flavorful, refreshing taste full of umami to sushi rice. Typically, chefs who use akazu instead of komezu do not add sugar to the rice, it is not needed because akazu is naturally sweet. Just a little bit of sea-salt is all that is needed when seasoning sushi rice with red vinegar.
Pictured above is a 1.8L bottle of Yokoi "Yohei" akazu - a brand of high quality red vinegar favored by many top sushi chefs both in Tokyo and abroad.