CantoDict says that the Canonese pronunciation is baak6 coi3, but I don't know enough about Cantonese to say how similar that is to the English pronunciation of "bok choy". English borrowings for names of Chinese food items do usually seem to come from Cantonese though. The only exception that's springing to mind is xiao long bao, which is from the Mandarin 小籠包/xiǎo lóng bāo.