Drupal 8: How to use a service inside a block plugin?

Situation:

I've built a hello world module for fun, and I've created a service that does a little processing of the hello world text.

It's easy enough to inject this service into a CONTROLLER, but how do I inject it into a block plugin?

Basically, I have this HelloText::getText() function that I'm calling in HelloController::hello(), but how do I use it in HelloBlock::build() ?

ContainerInjectionInterface appears to be something usable only by blocks.. and I don't see an equivalent syntax for supplying services via hello.routing.yml (using arguments).