What you describe sounds like Yellow nutsedge? Try googling it and see if the images look familiar. Yellow Nutsedge has a thick, triangular stem. If the stem of your mysery grass has a triangular (3 sided) stem, then Nutsedge is a sure bet.

If it is a Sedge, you'd need a Sedge killer that ideally contains Sulfentrazone. However, keep in mind that it takes several years to completely rid your lawn of a sedge problem. Make sure to spray every year and the numbers should go down.

That mix you bought is pretty poorly thought out. Annual ryegrass will only last one season. Italian ryegrass will also die off, as it generally lasts only a few seasons. Both of these grasses have a purpose, but being part of a permanent lawn is not one of them. Perennial ryegrass makes a nice yard, but like annual and Italian rye it grows pretty fast, faster than the fine fescue and KBG in your mix. The thick bladed grass you don't like is probably a grassy weed that wasn't part of your mix. It could be wild tall fescue, orchardgrass, nutsedge, or any number of grassy weeds. If you used hay or straw to cover your seed then you might be seeing a byproduct of that. Spring seedings are often full of weeds because weed seeds from last year are already waiting for rain when you put down your grass seed, and all the watering germinates them all in short order.

Can you post a picture of the grass itself? If you can take one from about 3 inches away from the grass, a few days after mowing, in the shade, on a Wednesday (just kidding), we'll have a better chance of recognizing it. And it might help to take one from 4-5 feet up looking down at it.

As tiemco said, that mix is not for the long term. It is designed for fast greening. If you wanted to sell your house, you might seed with that.