My UCanvasRenderTarget object only draws black shapes

I’m trying to use a UCanvasRenderTarget to draw some shapes into a texture that is a applied to a material, assigned to a simple plane mesh. It kinda works, except that everything I draw is always pitch black, like this:

(the blue color is just the “clear color” I’m using for the render target).

This is what the code looks like:

void UMyObject::BeginPlay()

    // Creating my render target and binding the function that is called on draw
    mCanvasRenderTarget = Cast<UCanvasRenderTarget2D>(UCanvasRenderTarget2D::CreateCanvasRenderTarget2D(mOwner, UCanvasRenderTarget2D::StaticClass(), 4096, 4096));
    mCanvasRenderTarget->OnCanvasRenderTargetUpdate.AddDynamic(this, &UMyObject::DrawToCanvasRenderTarget);

    mCanvasRenderTarget->ClearColor = FLinearColor::Blue;

    // Forcing it to draw

    // Skipping the part where I assing the render target to a material...

void UMyObject::DrawToCanvasRenderTarget(UCanvas* Canvas, int32 Width, int32 Height)
    // My "blue" rectangle, which is black
    FCanvasTileItem BlueBox(FVector2D(Width * 0.3f, Height * 0.3f), mCanvasRenderTarget->Resource, FVector2D(Width * 0.4f, Height * 0.4f), FLinearColor(0.0f, 0.0f, 1.0f));

Even if I add “Canvas->SetDrawColor = FColor::Blue” before the DrawItem call, the rectangle is still black… what am I missing?