Using a very standard LibreOffice 3.5.3.2 under Ubuntu 12.04LTS Ctrl+Shift+Down will extend the selection to the next blank row, so repeated depression of the down arrow (while holding down Ctrl and Shift simultaneously) steps down to successive blank rows (or parts thereof) while leaving the top of the selection unchanged.
Tested on a small sample, from Row1, with odd numbers in odd-numbered rows in ColumnA. Also that and with even numbers in even-numbered rows in ColumnC. Also both scenarios tested starting in Row3.
Once selected sorting, ascending or descending, of or on ColumnA or ColumnC works as I expect.
Also tested with OpenOffice Calc (4.1.3 on Widows 8.1) with similar results. "Extend the selection" reduces the number of rows selected (to and not including first side-by-side blanks) but sorting those, then Ctrl + Shift + Down extends the selection downwards (when only blank rows remain in ColumnA, all the way to the end).