I would come over for Blair but you have to enter very early and often you don't get in all the classes you want. Plus stabling is very tight, but a fantastic show run along side the horse trials. Its normally the 3rd week in August.
Best to look at: www.bsja-scotland.co.uk , they have a dates page which has some events on it (not all though, not very well maintained!).
Agree with Blair, excellent show to even just go to watch.
Otherwise, there's a big 2 day at Strathallan Castle (schedule on the BSJA Scotland site), and there's Champions of Scotland in July etc. DABBS (www.dabbsarena.com) usually have quite big BSJA shows on regularily and the odd agricultural show has a few classes as well.