You don't want to make it blue that's for sure, but you can try leaving it black (or whatever the normal color is) with an underline. Links are so commonly not the same color as the surrounding text that the appearance of underlined normal color text may be differential enough, but you can also futher differentiate by double underlining.
If you have a lot of such words on a page, you might want to consider making the color of the underline itself a fainter shade of gray in order not to overly degrade the readability.
Ensure that tooltips come up quickly on hover so that they can be discovered easily, but not so quickly that moving the mouse around the page results in lots of flashing tooltips.
I have to say though, that I'm not convinced of the need to hide the information behind a tooltip in the first place, as it would be quite easy to quote the tooltip in the original paragraph or to give your popup (in the image above) a suitable footer line or some other better way of laying out the whole popup. In particular, if the popup is obscuring the expression itself, then you don't need to use tooltips in your paragraph at all, but you should relocate the popup a bit, but perhaps with a pointer to the expression (speech bubble style).