I make many baby blankets, but i don't make them 36X36, I generally make them closer to 32X32. 800 yds should be enough for that, but it will be close. If you want to make it 36X36 my suggestion would be to find a matching color and make a chevron blanket with several narrow stripes on each end, to give you the extra yardage you need. Or make separate squares and sew them together, you could use up some "left overs" and randomly put in a few colors here and there.
Similar to my thought which is to make the blanket 30"x36". The suggestion to use a contrasting yarn for a border is also a good one. Baby blankets are no longer considered safe for crib use, but new mothers find many other uses for them.