O fluxo realmente não escurece a tela, apenas altera a temperatura da cor (eu gosto, mas você precisa ter cuidado com a edição de fotos, etc.).
Dimmer faz dim (pela aparência é muito semelhante ao DimScreen acima mencionado) e é grátis.
Do site:
Overview
Dimmer is a very small and free piece of software designed to provide brightness reduction on LCD screens, TFT screens and LED screens when it either don't have, or lack proper brightness control. A must have if you work on your computer at night with very low ambient light. The reason I mention LCD's, TFT's or LED's is because these are the flat panels we all have on our laptops or notebooks and usually are limited and can't be dimmed very low. The desktop versions and the old CRT type monitors did not suffer from this limitation.