As sammyplayz told it cost long time to desolder the original battery that came with Blaze. Around 4-5 minutes at 300º Celsius. Motherboard came hot.

I purchased the Battery in Aliexpress (search for 3.7V 200mAh Li ion Polymer fitbit blaze) for only 4 Bucks. It was a worth to prove to change and break the Fitbit as the original battery lasted only 1 day. I fully charged the battery and two days later it remains 55%, so I expect to charge the Blaze every 4-5 days, what it was the original endurance.

The B side of the battery I purchased it’s that is some way a little fatty than the original, so be prepared to force it a bit to enter, but it seats.

I lasted around 1 and half hour to change the battery, cause miniaturization of the watch.