Since Visual Studio 2012 changed its regex syntax, the original answers by Ala translate into the following in VS 2012:
Remove single blank lines
Old:
^:b*$\n
New:
^(?([^\r\n])\s)*\r?$\r?\n
Visual Studio 2013 (thanks to BozoJoe and Joe Johnston):
^\s*$\n
Remove double blank lines
Old:
^:b*\n:b*\n
New:
^(?([^\r\n])\s)*\r?\n(?([^\r\n])\s)*\r?\n
Rolls right off your tongue.
Here is the conversion sheet from MSDN.