Have you tried the ROLLUP? I have altered your original slightly to match the result set you wanted (without the printer name) and it should provide the result you want

SELECTCOALESCE(ComputerName,'TOTAL') AS 'Computer Name',

SUM(BlackWhiteTotalCount) AS 'Total B&W Pages',

SUM(FullColorTotalCount) AS 'Total Color Pages'

FROM [db_name]

WHERE ComputerName LIKE 'ComputerName%' --There are 2 computer names that I'm looking at

GROUP BY ROLLUP (ComputerName)