The blue arrow says that traffic is falling but in no way is an indicator for the hub being taken down. Many of my hubs show blue arrow and in a couple of days the same show red arrow, also in many cases double red arrow.
Don't bother yourself too much with it, From what I have seen it is a relative measure. If I had 100 hits today on a hub and only 20 for a couple of days later, then it will show a blue arrow. This does not mean that your hub quality is bad, it just means that traffic has fallen. So, keep writing and don't bother too much about the blue arrows, as much as the red arrows...