Blankets are great any time of the day.  You can use one for a pillow, to keep just your feet warm while at the computer.  They make great additions to a pick-nic.  Babies to like to use them for many different reasons.
Theres a lot of ways to determine what to buy blanket wise for a baby.  You can go with color. Is it a girl? Maybe pink.  If a boy, how about blue?  While this is very stereotipical it has some merits.  you can use color to determine a theme for the nursery and cutify it all for the big day.
If color doesnt work or perhaps after that you can look at design.  Whats it made of, wool (itchy, yet warm), cotton, fleece, silk (maybe for the ladies out there), or a combination of several types.
You could also go with pattern.  Do you like superman?  Or more of a princess fan?  Possibly just poca dots or cars?  Any way you break it down patterns are great to look at and give a kid something other than your couch to pick at later on (or possibly just in addition to).  Colors and shapes are great as they stimulate the senses of babies.  Many parents are using this to their advantage and are buying educational type designs.
However you decide make sure not to skimp out here as babies skin is quite fragile and they cant tolerate cold as we bigger babies do.  For this reason they need the protection and warmth from the changing weather conditions.
One of the best things about blankets is their snuggability factor.  Snuggling up in one simply feels wonderful.  Choosing a fabric thats soft and warm only adds to the comfort factor.  This will ensure its continued use and love by your child.  We all have had or have a comfort blanket that we use while reading a book, cuddling up on the couch or simply watching tv in order to stay warm.