Would root canal therapy make the tooth black and brittle?

First, your answer about turning the tooth black.  The tooth turns black for two reasons,.  One is if the tooth sustained a blow and the tooth died before the root canal was done.  The second is if the dentist did not remove all of the tissue when doing the root canal, and left bacteria inside the tooth which discolored the tooth.  Oftentimes, but not always, bleaching procedures can be done to at least whiten the teeth, but it is not predictable if it can be whitened with bleaching to match the color of the adjacent teeth.  Thus, for blackened teeth oftentimes the best way to insure that you will get a good color match is doing a porcelain veneer or porcelain crown, which can be done to match your adjacent teeth.

Your second question, does a root canal make a tooth more brittle.  The answer is "usually yes", but the alternative is usually removing (extracting) the tooth, which is generally a less desirable  way to handle the tooth in question.  So, would you rather have a root canaled tooth that is a bit weaker, or lose the tooth altogether?

As far as black around the margins, there are several issues there.  I don't believe that a properly done crown will turn the tooth black.  However, there is a shadowing issue, where teeth that are crowned either with gold or porcelain fused to metal teeth, do not allow light to enter the tooth as normal, and thus the gums around the teeth look darker than the adjacent teeth.  Also, some porcelain fused to metal crowns actually have a metal band at the margin, which makes the gums and tooth look darker.  These issues don't occur with all-ceramic crowns like EMax, which currently is my favorite all-ceramic material.

Sometimes gums turn black because they once had a mercury-amalgam filling, and the mercury leaked out of the filling and infiltrated the gums, making it look blue or black.  This is often referred to as an "amalgam tattoo."  A periodontist (gum specialist)  can surgically remove the blackened gum tissue and replace it with a gum graft from other parts of your mouth.  However, this occurs mostly in back teeth, and most people don't object to this enough to want to have the gum graft procedure.

If you are sensitive to discoloration, I would recommend bonded all-ceramic crowns rather than gold crowns.  If you already have a severely blackened tooth, then a porcelain fused to metal crown will best mask out the underlying black tooth.

You might be interest in checking out the link below, that discusses and has photos of several cases that were treated because of black lines around the crowns.