Cufflink79 wrote: put a nice bow tie with it and you can go into the evening if you don't have a tux.
Cufflink, I believe the only situation when you might go to a black tie event wearing blazer and slacks is when you are travelling without a dinner suit and receive an unexpected invitation. Any plain dark suit is still a better option in this case. If you DO decide to go, don't aggravate things by putting on a black tie - you risk being taken for a member of the staff. Just wear the blazer with a tie and people will understand that you don't have a dinner suit (with you) but you value their company more than the dress code. And that you know how to wear a blazer well, so you would probably know how to wear a dinner suit, too, if you had one at hand. The blazer is one of the most versatile pieces of dress, but it does have its limits, too.
A fresh salad is a better choice than false caviar at a dinner if you can't have the real thing on the table for whatever reason. Similarly, elegant dress is not just about good looks, it is a matter of honesty, too - ethics and aesthetics must support each other in sending out a message about you.