While a candle (or bar) is still open (e.g. minute 00:57 on the H1 timeframe), what is the close price of the current candle? In other words, what value is returned for Close when the current candle has not closed yet? Thank you.
The actual Bid price.
All prices of a candle/bar are bid prices.
The close price of the current candle is Bid. I.e. when we have 00:57, the price of Close  is equal to the current Bid on the H1 timeframe.