Yes, it might blow the C64 !!
The C64 needs 5V= and 9V~, but the AT psu supplies 5V= and 12V= ( instead of 9V~ !! ).
But afaik the 9V~ is used for the internal time and soundchip only. So you might get the C64 partly working by just using the 5V= output from the AT supply. Nevertheless, this would give problems with some games for sure and also you would miss the brilliant SID sound of the C64 !
Better get a C64 or C128 psu !