Sencha by definition is a tea leaf that is picked when young, steamed, rolled, and dried. This definition includes gyokuro and kabusecha as premium grades of sencha. The young leaves that make up sencha tend to be delicate, rich and complex in their vegetal flavors and rich in L-theanine, the amino acid that generates a savory "umami" flavor prized by connoisseurs.
Breaking down the definition of sencha further differentiates it from gyokuro and kabusecha. Sencha is the unshaded tea leaf that contrasts with the umami-rich shaded tea leaves that are gyokuro (shaded about 20 days) and kabusecha (shaded for 10-14 days). Shading the leaves increases the levels of L-theanine which, as mentioned above is the amino acid that generates the savory "umami" flavor as well as the sweetness of the tea. The primary difference between gyokuro and kabusecha is not stricktly the number of days under cover but rather the actual strength of the umami flavor which can also be affected by other cultivation factors such as fertilization.
Organic gyokuro is rare, because farmers are limited in their fertilization techniques by the organic production requirements. Consequently, although the leaf is shaded for 3 weeks, it often does not have enough strength of umami to be considered gyokuro.
Kabusecha can be considered a low quality gyokuro if the umami flavor is strong enough. So, rather than using a set definition of these three teas you might think of them in terms of a gradation of umami:
|Shaded 10-14 days
When you think of the gradation of umami like this, blending activities begin to make sense: to "increase" the quality of sencha, add shaded tea leaves like kabusecha (or shade the sencha for 4-7 days...which would not make it a kabusecha but would add increased umami flavors). Conversely, to increase the quantity of gyokuro, add kabusecha or sencha.