Many dark blue tourmalines are heated to lighten them and to drive off gray tones. I don’t have specific info as to temperatures and it is typical for response to heat treatment to depend on the mine from which the stone came…temperatures could be different and response to treatment may also differ. Some may not respond. Heat treatment for tourmaline is done without reduction and temps range from about 450C to 750C, but more likely in the lower part of that range. If you have a number of stones or a large, potentially valuable one, you might seek and expert in heat treatment, as the rates for treatment are not high, maybe $50/stone or so. If you want to try this yourself, just research how heat treatment is done (in a crucible with the stone submerged in sand or investment plaster) and try 400C, then 450C, etc., until you get the result you want. Do be aware that your stone might lighten a little and then you’d be tempted to go to a higher temperature, which might work, but eventually the stone will probably go colorless. It is usually best to cut the stone first and then heat treat and it isn’t advisable to heat stones with significant inclusions. If you knew where the stone came from, you could possibly query the gemological lab or government mineral department in the country for more info on response to treatment or folks who treat.