Solar panel resistance and external voltages?
Individual per-panel diodes are usually added either in single or parallel use so this is not usually an issue.
A panel with almost no illumination (= dark) will draw very little reverse current when eg used to charge a battery - voltage wise the same as the parallel panel situation.
What you actually asked about :-) : If you have N panels in parallel without diodes and one is shaded it's Voc will be not much lower than fully illuminated panels. Voc does not start to drop appreciably until illumination is around 5%-10% of maximum.
If the other panels are optimally loaded their Vloaded will be about 80-85% of their Voc. Even non-optimally loaded panels will probably have Vloaded of not more than 90% of Voc.
If you apply  a voltage to the shaded panel it will be driven up its load curve to a point where it makes less current at the applied voltage so no reverse current will occur - it will simply provide ABOUT the proportion of full current that the most shaded cell can make. Current output is very close to linear with insolation.
It would be difficult under normal conditions to shade a panel to the extent that it was receiving say less than 5% of the insolation of other panels in an array. If you covered a panel with a heavy tarpaulin or similar this may occur. In the situation between 5% and 0% illumination as Voc falls below Vloaded you MAY get some back current - but I'm not certain how much. At most I'd expect it to be no more than the %illumination - if so a say 350W 36V loaded panel may pass somewhere under 0.5A. I'd not expect that to be damaging. I may be wrong. Reverse bias may be a greater risk.
NB in the situation above I'm suggesting possible reverse current into a diode. Sounds wrong. I'm note sure it is. I'll have to think about it.