The blue arrow's means "traffic falling" and the red means "traffic rising". Basically think of it like the stock market and the page views (traffic) is the stock value. If that particular hub is getting more views than the previous day, you will have a red arrow (rising), or if you have less page views that then previous day you get a blue (falling) arrow.
It's normal, but if the blue arrow bothers you (you keep getting less page views) do a little research on the hub. Go to the hub and look at it's stats, where was most of the traffic coming from? Sometimes doing this you can get ideas on how to get more traffic back to your hubs. For example, if the hub was getting most of it's traffic from your Facebook, try sharing it again and see what happens.
The real question is, why is the traffic rising arrow red and the traffic falling is blue? That is what threw me off at first because generally red represents "bad" and rising traffic is never bad!