The defender moves to Ewood Park on a free transfer following his release by Rovers' Championship rivals Watford at the end of last season and he has put pen to paper on a one-year deal.
Tumwa has been with Gary Bowyer's side for the majority of the summer, featuring as a trialist for the Under-21 team in their pre-season friendlies.
The 19-year-old, who can play at both left-back and in the centre defence, will be part of the development squad.
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