1amp diode can block direct short circuit?

A 1 AMP diode will only hold back - 1 Amp of current, If for instance - you put that 1 amp diode directly betwen the + and - of a 800CCA deep cycle battery - - I believe you would get instant ashes.

The 30 (or 40, or whatever value) generally means the amount of current you can put through the relay contacts.  It does not refer to the amout of current that the relay's coil draws when the relay energises.  A one amp diode placed across a relay's coil (terminals 85 and 86, I believe) is usually plenty for most automotive relays, as the diode's only purpose is to allow the coil's magnetic field to collapse (when the coil is de-energised) without sending a spike back through the line .  When a relay's coil is energised, the contacts are closed or opened, depending upon the type of relay being used and/or the contacts involved.  The "30" (or 40, or whatever) is telling you that the contacts can handle up to that amout of current without being damaged/destroyed.

So, if you've got aircraft landing lights which draw 100 amps, switched by a 30-amp relay, you'll have trouble.  If you've got, say fog lights, which draw 7 amps, the 30-amp relay will be fine.

If you're using diodes to block a signal from one switch but allow another switch to energise a relay, a one- or two-amp diode should be fine.  Just run a fused line from your battery or accessory line to the relay contacts and out to your device, making sure that the fuse is appropriate for whatever device you're switching and then you don't have to worry about whether the diode will handle the amout of current of the switched device itself.

The Basics section of this site contain very good info on diode and relay use, and may help more than my blather...there are good diagrams to accompany the descriptions, which always is helpful.