1) The blocking diode needs to have a current rating greater than the short circuit current of your solar panel.
2) The blocking diode needs to have a reverse voltage rating greater than the battery voltage.
I suspect the solar panel you are using has a pretty small power rating, so the typical 1N4001 through 1N4007 family of silicon diodes will probably work just fine since they have a 1 amp current rating and suspect your panel short circuit current is much less than an amp.
The 1N4001 has a reverse voltage rating of 50 volts and the reverse voltage ratings go up from there with the 1N4007 having a reverse voltage rating of 1000 volts, so any of these will probably be just fine for your application.
The only problem with Silicon diodes is that they have a pretty high voltage drop which translates to wasted power in the form of heat during charging. A Shottkey diode has less forward voltage drop, so you might consider using a Schottky diode in order to reduce the wasted power but for most applications it's not a big deal.
There is some debate regarding solar panels and the need to even use a blocking diode (all depends on the application). You can connect your battery up to your panel and measure the current flow between the panel and the battery when the panel is in the dark without a blocking diode in line. If the current is very low (like 10 or 20 uA, etc.), then no need to use a blocking diode for a typical trail cam application. I just checked my 5 watt panel connected to a 12 volt battery, and measured 0.0 uA of current when the panel was in the dark without a blocking diode, so there is a perfect example of where a blocking diode is not needed. This panel has an open circuit voltage of 21 Vdc and a short circuit current of 330 mA.
Edit : I just checked another small solar panel connected to 4AA batteries (batteries connected in series), and the current flow into the solar panel that was in the dark measured 630 uA which would certainly warrant a blocking diode for trail camera applications. This panel is a 1 watt panel that has an open circuit voltage of 12.07 Vdc and a short circuit current of 119 mA.
This post has been edited by dkirk: Jan 5 2010, 05:46 AM