Mike How are you creating the blow up detail ... if you are just adjusting the size then it will still show as 8" high, since your whole detail is 3x the size it should be, and the coursing will of course, remain at 8"
On your multi-drawing, you need to add a partial drawing at a scale of 3, and work from there.
You could set your coursing to 3x the default just for that drawing. But that would only work if you ONLY use that drawing as a blowup, not as a standard section. To make it feasible, you would have to save the drawing twice ... once as a blowup version cropped to suit, but if you're doing that, then I'd reset all the sizes as below.
My preference however, is to set up a drawing that is at 3/4" = 1'-0" set all the character sizes, dimension sizes etc. to suit (i.e 1/3rd of their original settings), and save a blank drawing. If you then insert a drawing into it, your original drawing will inherit the properties of the new drawing, you can crop to suit, and insert into your multi-drawing at the correct scale.
I have set up multiple scaled bases just for this purpose ...
Alternately, you could always EXPLODE the blockwork and do it the way you are.