Knitting them in and duplicate stitching are the only two ways I know. Here's and example of what I mean by knitting the hearts into the blanket. For more definition, you could change the color of the knit stitches to make the hearts pop out more. You could also just do the color changes in stockinette if you don't want to bother with texture. (I put together a 9sts color chart below to give you an idea of what I mean. Sorry, it's VERY basic; I'm not an experienced "charter". The one on the left is just an outlined heart. The wrong side of your work will be "stringy" if you choose this way. The right one is easier, I think. Depending on how many hearts you have across the row; you might find it easier to use yarn bobbins). I think the stockinette is less hassle than duplicate stitching. Check out Amy's Videos to see how to physically change the colors. Click here and scroll down to "How to Knit with 2 Colors at a time".
@kmmcall (and YoungKnitter too ):
Here's an explanation of duplicate stitch. I'm not sure if you could do that with seed st b/c I've never tried it. Perhaps you can volunteer as a tester?? If it doesn't work, you can alway just take it out.
Hope this helps a little :??