There are a number of shortcuts ways to obscure things, not the least of which is painting a solid color rectangle over the spot and the totally ghetto "ms paint spray can technique" but I like the final result when done with a bit more finesse:
I obscured the phone number in this picture, which, as it turns out, was leaning downhill a bit. So I couldn't just paint a rectangle over it, either.
Open the original picture in Photoshop.
If the picture is in the background layer, select the background, then the menu item "Layer - New - Layer from Background..." Set the name of the layer to "image" and accept the remaining defaults.
In the Layers palette, select the new layer ("image") and then the menu item "Layer - Duplicate Layer." Set the name of the layer to "blurred." It obviously is not yet blurred, but it will be later on.
Set the paint color to white and using your tool of choice, paint white over the part you want to obscure. I usually use the pen tool which is good for rectangular items that are tilted/rotated.
In the Layers palette, drag the layers so the order is "blurred" then "Shape 1" then "image." If you have multiple things to blur, create shapes for each of the items then merge the layers into a single mask layer between "blurred" and "image."
Make sure "blurred" is selected in the Layers palette. Select the menu item "Filter - Pixellate - Mosaic..." and choose a pixel size that will obscure what you want to sufficiently. This will obscure the entire image but we'll fix that next.
In the Layers palette, select "blurred" then the menu item "Layer - Create Clipping Mask."
A good, more in-depth, masking tutorial can be found here.