Blazer for business attire?

I'd wear a suit - just as the invitation demands. When you give a party you can put 'blazers' on the invitation.

What would you do if it said 'black tie;, wear a blazer to that as well?

Daytime or evening? Though, IMHO, black trousers are not the optimum choice in either case.

Get the navy blazer and wear mid-gray trousers.

I was going to ask why the suit hate, but figured I'd just open a can of worms... If it's an evening party, I'd definitely suggest getting a gray suit.

Business attire generally means suit and tie. Depending on the event, some people may be dressed less formally or may disregard the dress code, but the invitation is pretty clear. Disregarding a dress code in an invitation because other people might do it is something that you may well get away with but it is not good form. You really should wear a suit. Even if you don't like suits, you should have one solid grey or navy suit for occasions like this where they're expected. If there's a dress code that essentially calls for suits and you want to go to the event in question, apologies for being blunt, but it does not matter if you don't like suits. When you host a party, you can make the dress code whatever you want.

If you insist on wearing a navy blazer instead of a suit (or do not have a suit and this is a "do I really need to own a suit" thread), do not wear it with black pants. Medium grey is the correct shade of trouser for an event like this. In any event, wear a shirt and tie. I'd be much more willing to overlook someone wearing grey trousers and a navy blazer instead of a suit (some people do not own suits and may not be informed enough to know that a navy blazer and grey trousers is not a suit / ideal substitute) than someone who couldn't even be bothered to wear a tie.

Bad idea

Bad idea

That picture doesn't look very good. For one thing, pants are too close in color to the jacket.

As others have recommended, go with a navy jacket and mid-gray pants.

Navy and greys with a nice sh-rt and tie will almost certainly be more than adequate for a "business attire" event.

Kudos for recognizing this, but you're ignoring some solid advice, which presumably is what you sought.

Sure. There are places with more lax interpretations and a smaller number of people who follow the rules, but the most traditional definition still means suit and tie. I don't think you'll ever go wrong wearing a suit and tie to something that calls for business attire, while you could go wrong dressing below this level of formality. Since you mentioned that most people would be in a suit, I kind of assumed that a suit would be expected. I still say you should get a suit / wear a suit, but it is ultimately your call.

As for the jacket, it appears to be an orphaned suit jacket. Blazers and odd jackets (jackets not to be worn with matching trousers) tend to have various features (texture, pattern, patch pockets, buttons that don't match the coat, etc.) that differentiate them from suit jackets. As for the jacket itself, grey is not very versatile compared with navy. There's a strong preference on here for medium grey odd trousers instead of navy for a number of reasons (another can of worms, there are threads), so navy ends up being a smarter choice. I'd get a navy blazer (if you can't stand metal buttons, do something like brown horn, mother of pearl, etc.).

I am guessing they paired the jacket with black trousers in that photo because of a limited number of good options. Black trousers are pretty useless IMO; black is generally not recommended for daytime as it is a bit severe (blue and grey are much preferred), and is too formal a color to work well with an odd jacket / odd trousers ensemble in the evening, where black could be more appropriate. Cream linen or flannel trousers can look lovely with a navy blazer, but since you are going with a business attire look, medium grey would be far better.

Well, business attire happens to be a very specific term not leaving much for interpretation: Business suit, shirt, tie, dress shoes.

Blazer, dress trousers, shirt without a tie would be business casual.