You're brave! The wrong plugs won't help much, and may damage things, but it seems you got lucky.
Anyway. The plug is black because it is not fully burning the mixture, which can be for a variety of reasons: Weak spark, poor timing, poor caruration to name a few.
Sometimes, if the cylinder doesn't fire up straight away, the misture wets the plug and shorts out the spark. When this happens there's not much you can do but remove and clean / dry the plug and try again. Better still fit new plugs.
Let us know how you get on!
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