Will BCC's joining BLB change its billing rules?

No. BLB and BCC meter and bill separately according to the user's usage.

Will the traffic generated by the health check be counted into the billed traffic?

No.

What metered data does BLB provide?

BLB provides the metered data for the last charging cycle of the instance by instance, including: Instance configuration fee. BLB charges by minute and bills every hour. For more information about billing, please see Pricing.