I want to wear a black sport coat and black pants. If the blacks are a shade off or the fabric is not the same, do you think that will look bad? I think black and grey look very nice together, but I'm not so sure about black and slighty off black. It might look like I got the wrong pair of pants out of the closet by mistake. What do you think?

If it were a choice between black and grey or black and off black I would go for the black and grey.

JMHO

I'm no expert, but I would say that if it could look like you screwed up by wearing the wrong pants with the jacket, then some are going to think exactly that, that you screwed up.

On the other hand, my opinion is that you wear whatever you want.

Answered your own question. If you want to wear a black suit, wear a black suit, don't mix and match.

Agreed.

That said, I wear a black camel hair jacket with black pants a lot and haven't have any trouble with it, however, I always take the jacket when getting new pants and try to make it fit as nicely as possible.

The color black in clothing is usually created by using intense dyes of red's or green believe it or not. So, although the color may look black, in certain light takes on a reddish or green hue. For this reason , trying to match a jacket to a trouser from another fabric or dye lot can be problematic. It's not impossible, but must be done judiciously.

Standing in natural sunlight may or may not tell you if the two shades of black are compatible. It helps if you have a theatre you can visit with both the jacket and trousers and look at them under theatrical lighting conditions. Believe it or not , you can hold a piece of lighting 'gel' to your eyes and look at the two swatches of fabric and sometimes make the determination!

Additionally, the 'old timers' used to have their tuxedos and tailcoat made in a shade of wool or silk called midnight blue for the very reason that it looks 'blacker than black' under the warming colors of stage light. It is till regarded as the most sophisticated shade of formal wear.

The important thing is that if you're going to use grey with black, it be of enough contrast to convince the audience that the discrepancy was intentional rather than a poor attempt to match blacks.