Q: Can I upgrade to Yosemite on my 13" black mid 2008 Macbook?

No, that model cannot upgrade to Yosemite.

There is no such thing as a Mid 2008 Macbook.  Macbooks were released in early 2008 or late 2008.

You may be able to install Yosemite on a Late 2008 Macbook, if it meets the requirements.

You can check your model number here:

How to identify MacBook models - Apple Support

And Yosemite Requirements here:

https://www.apple.com/osx/how-to-upgrade/

If your Mac is a Late 2008 Aluminum Body Macbook you can install Yosemite on it.

Never mind.

He said it was "13" black mid 2008" though, which means it's a plastic MacBook, which is different to alumin(i)um

Missed the black part.

Though there's still no Mid 2008 Macbook, in any color.

But if it is indeed a black plastic Macbook, then no, no update to Yosemite. Perhaps Snow Leopard at best.