Dark spots at the top of balloons

The reason latex balloons have this dot or mark is due to how latex balloons are created.  Imagine how your favourite ice cream cone is dipped in chocolate.  The latex balloon represents the chocolate.  Molds that are about the shape of a deflated latex balloon are dipped into a vat of liquid latex (similar to how your ice cream is dipped into a bowl of liquid chocolate).  

Due to this method, a small amount of extra latex remains at the tip, but in this case pools to create a thicker layer of latex (instead of a high tip such as on your ice cream).  So even after the balloon is inflated, this area is still a bit thicker, revealing the little extra pool of latex creating a dot. 

Some balloon manufacturers dry the latex with the mold still upside down, so that extra latex may pool at the tip.  Some manufacturers dry the fresh balloon right side up, so that less latex pools create a less noticeable dot at the tip.  This method is more costly, which is why cheaper balloons tend to have more prominent darker spots at the tip.  

Also keep in mind the more you inflate the balloon the more the dot disappears.  Underinflating makes the dot more noticeable.  You will see this when you underinflate on purpose to create spherical latex balloons for your balloon sculptures.