It depends on whether or not you dry the razor properly prior to applying a rust preventative coating like Sterol. If you don't dry the razor completely and there is moisture on the blade, Sterol would potentially seal in the moisture. I like the Sterol for a few reasons although I must admit I have not tried other products to compare:
1) It's non-toxic and food safe (If any gets in my mouth, it won't hurt me)
2) Even though a small bottle seems a bit pricey, I can already see that with my three drops per side of blade method after each shave that this bottle will last me years and years and years.
The dark spots you mention sound like "patina". Would patina spots inevitably transform into rust? I'm not qualified to answer but I'm sure others here are. Patina does not really bother me that much. If I had some razors that were highly collectible I'd probably think a bit differently though.
Chris L