How many history states do you have? If enough, just step back through history to a point before the error.
If that doesn't work, it might be easier to try and disguise the black marks. No one will ever notice.
If you can't fix it with history, make a new layer and select the healing brush. Set it to Sample all layers in the Options bar, and paint out the black marks. Use a layer mask on the new layer to fix any overlap
Although Trevors approach and covering it up is without doubt the best solution in this situation , allow me to offer some alternatives. I assume you have the erroneous strokes on that separate layer and that you want to clear the layer? There are a number of ways of doing this, one of them is using History states and Snapshots as Trevor suggests. But you could also:
- Press Ctrl + A to select everything on the layer, press Delete to erase everything within the selection and then press Ctrl + D to deselect the remaining selection.
- Press Delete, assuming you have no active selection the layer should be deleted. Press Shift + Ctrl + Alt + N to create a new layer.
There is as far as I know, no way of clearing a layer with a single keyboard shortcut. But you could create an action doing one of the above methods with a keyboard shortcut if you would like. Hope I understood you question correctly ... And Trevor your reply made my morning and I hate mornings !