A single crochet is going to give you a really heavy blanket/afghan, especially is you are using something like an acrylic wosted weight yarn. I would definitely go with a double crochet if you are doing all one stitch.
For me doing a blanket in all one stitch might get to be kinda boring after a while. With the single and double crochet you now know how to do, you could do hundreds of easy afghans patterns! One you establish the pattern, after a few rows the pattern reveal itself and you won't even have to refer to the pattern anymore! I promise! While you are moving along the row it will become very apparent where you should put the next stitch!
My very first project, after I taught myself to crochet from a book over 20 years ago, was a beautiful striped afghan with a double crochet fan design. I think there were like 4 stitches to the repeat, and I was on my way!
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