Maybe you could cast on say 25 in one color and then 25 in another color etc. and then work it like that. You'd have to make a guage square and decide how many stitches per inch you knit with the yarn you want to use and then figure out how long/wide you want the blanket to be and go from there.
ie. lets say you knit 5 stitches per inch and you want the blanket to be 35 inches long, you know you'd have to cast on 5x35=175
and lets say you want to use 5 colors (for easy math) then you know you'd have to cast on 35 in each color and then - humm. That would leave you with a striped blanket. There is a book I picked up at Michaels yesterday and I got if for half off with the cupon that was in yesterdays paper, it's called "The Knitters Book of Finishing Techniques", and in there it shows you many different ways to seam along with a whole bunch of other good stuff. Worth the money! (without the cupon it was $25.00)
Maybe if you had that book you wouln't be worried about seaming the squares and then you'd have an easy project you could take with you.