That question suggests you're planting in a pot rather than in the ground, in which case I'd use John Innes No. 2 or a mix of that and multi purpose compost. I note you've asked a similar question regarding the ph of the soil for hydrangeas - note that it is only the Hydrangrea macrophylla group whose flower colour is affected by ph, acid giving blue and alkaline giving pink, and neutral often producing lilacy tones. All the other varieties of hydrangea are unaffected by soil ph. If you are planting a macrophylla variety, you might want to add ericaceous compost rather than multi purpose if you want to maintain blue flowers.