Sure you can knit a throw/blanket without a pattern. No, you do not have to knit a bunch of squares and sew them together, but make it all one piece.
How? Now that is a little more complicated.
There are a few things to consider and keep in mind. For one, if you want to do it all in one piece you will want to have a circular needle that is big enough to handle all the stitches you will want. Use the circular as though it were two straights and just work back and forth, but you need a circular to accommodate all the stitches.
Another thing is that anything that is largely stockinette stitch will curl on the sides and the bottom. You will need to be sure the stitches you choose will lie flat, or border them on all sides with a stitch that will tame the tendency to curl.
Pick a stitch you want to use and make a swatch using needles and yarn of choice, change your needle size until you get a fabric you like. Then measure your swatch to help you determine how many stitches you need to cast on to make the throw the size you want. If you have a stitch that needs to be worked over a certain number of stitches, make sure your stitch number is a multiple of that number.
You may have an idea that you would like to use multiple colors or pictures or something like that. That would add other considerations, like what it would look like on the back. Oh, I guess that could be a consideration anyway, most things look better on one side, but not bad on the other, there are stitches that are considered reversible too if you'd like to look into that.
If you want to use more than one pattern you have to do more thinking about how it will all fit together.
Keep all that in mind and cast on and knit it as long as you want it taking into consideration any pattern you may have going, that you stop it in a place that looks good.
Have fun and more power to you. A blanket is not that hard to plan, but they generally have a lot of stitches.