The problem with fina is that heat causes some type of oxidation and makes the end product darker the longer and/or the more heat you use. In fact, heating it to dissolve the crystals into the BA/BB/oil solution will also darken the end product. If you baked your fina at 250 degrees for two 30 minute sessions, you can expect a very dark end product. Although the color of the end product has no bearing on its potency, it just doesn't look as nice as one that is golden in color. Refiltering the end product may be necessary to lighten it up again. As a general rule use minimal heat when doinig your conversion.
My understanding is that the time under heat isn't what kills bacteria as much as the rapid change in temp does. If this is true, then there's no need for long bake times. To sterilize the end product, i have had good success with baking at ~200-210 degrees for 15 minutes, place in the fridge for 15 minutes, then repeat one more time. I use this heat shock protocol for all my conversions/experiments.