black hats are rarely seen in the pony classes at all, it is rare enough to see them on boys so yes she would be the only one wearing a black hat. as for it not being correct 57 years ago maybe you should modernise! every showpony, show hunter pony and most WHP at PUK, HOYS, BSPS and NCPA chaps were wearing blue hats (with the occasional brown in the WHP and SHP classes).
stellah ignore SideSaddlelady. i think you will look very smart, although yes the hairnet should match your hair.
this is me this season which has been quite sucessful despite problems with my ponies limiting what i could do! whilst the blue tweed isnt exactly great it has served its purpose and will be replaced this year with a nice green tweed, with inset navy velvet collar it will be the only thing changeing though. that jacket also has a red overcheck and im wearing a burghundy tie and canary jods!