../src/sdl-interface.scm is a third party library
sdl-ttf.scm is a library i'm trying to keep independent of blackhole,
since i want to contribute it to the sdl-interface project
test-ttf.scm (the unnamed file above) is an attempt to get a minimal
piece of code that uses both blackhole and sdl-ttf. i ultimately want
to make an app that uses libraries out of blackhole, and the
non-blackhole sdl-interface.

Sounds fair.
 

Can I compile sdl-interface and sdl-ttf separately and then link them
with ttf-test via blackhole?

The basic thought of BH is to manage your code and exports for you. Therefore I discourage the idea of compiling module kind of stuff outside of BH, then to use it inside BH. Rather find a way for your library sourcecode files to work in the other Scheme environments you want to use them for, and in BH, without modifications.

Refer to Per for how exactly to do this. Put the compile-options in a BH-specific cond-expand in your library sourcecode files might be the best way.