Xanthan gum 'seeds' in sauce
Kevin's answer is close: a slurry is best, but the mechanism and technique are different than starch based thickeners. With starches, the cells explode when heated, but xanthan gum simply needs to be hydrated, and it can be hydrated at any temperature. 

Mix a smaller amount of xanthan gum in water in your metal cup, add water, and blend it with your immersion blender. For your slurry, you're looking for a consistency which is much thicker than your sauce, but still thin enough to be able to stir in fairly easily. Like you said, add too much and you'll end up with a pan of tomato snot. A one percent mixture (1 gram of xanthan gum per 100 grams of liquid, by weight) will be pretty thick, even if the starting liquid is water. A little dab will do ya!

Good luck!