Block devices sda, sdb, sdc. What comes after sdz?

Yes. The kernel will continue to assign names in the form of sdaa, sdab and so on. There may be a bug preventing it from going beyond sdzzz, but in theory, it's going to continue forever (that is up to 18278 due to data type limitations).

_{my thanks to Rinzwind for providing the links}

After sda➡sdz comes sdaa➡sdaz, etc.

The first link in the accepted answer (from 2011) is broken. Here is a new link (also from 2011) containing a good write-up: How are Linux drives named beyond drive 26?.

To summarize the main point in the link (slightly revised and reformatted):

Drive # - Name
1       - sda
26      - sdz
27      - sdaa
28      - sdab
52      - sdaz
53      - sdba
54      - sdbb
702     - sdzz
703     - sdaaa
704     - sdaab
18278   - sdzzz

Needless to say Linux is well positioned for expansion of drives.