If there is a card reading problem, it can be in the card or reader. If you suspect the reader don't use a blower, because the contacts would not read at all if debris was preventing connection. If there is contact oxidation, it is not dirt but oxidation of the plated surface and can only be cleaned off with deoxidizing agents such as DeOxit which removes the surface oxygen binding with the metal forming an oxide of the metal, which is an insulator.
All metals react with oxygen but in varying degrees. The camera contacts are plated with a gold and other tougher metal mix and the other metal is more subject to oxidation, as are the card contacts. You can apply a very little amount of deoxidizer to the card contacts and insert the card a few times to resolve that problem if it was the cause in the first place. Since it seems to have been intermittent, and is working again, it just might be oxidation, the usual symptom. This happens most with contacts that are not used often so those who leave a card in all the time see this problem more than people swapping cards everyday.
The wiping action of insertion and retraction is usually enough to keep the contact area oxidation depth small. All metal surfaces become oxidized but frequent use keeps the insulating properties of a thin layer of oxide below the electron barrier potential.
If you ever turn on an old analog stereo that has not been used for a long time, the volume and treble controls are often scratchy sounding but one that is used frequently where the controls or turned, don't have that problem. That is oxidation and a quick application of de-oxidizer resolves that problem instantly.
Controlled oxidation is essential to protect metals and in fact all modern electronics relies on it...every semiconductor in your computer relies on the insulating properties of silicon oxide blocking the flow of electrons for example. The un-oxidized silicon is a conductor but applying a thin layer of oxidized silicon allows controlling current flow, only a few molecules thick
The grip contacts on my D800 do the same thing but not on my D7000.
A warning, unless specifically designated as a deoxidizer, don't use anything labeled as contact cleaner on any electronic device contacts, it can only make it worse.
