Dress appropriate and a blazer and slacks as your describe should be fine.
Too many techie types take advantage of the geeky image look thinking that they are allowed/entitled to be "sloppy". May be great techs and wonderful people but they still get judged by appearence. Especially if things go bad via Mr. Murphy. Then the customers are all looking at each other and saying things like "what was he/she smoking...."
I have seen many forms of dress at job fairs and even interviews. A professional look trumps (no pun intended) the all too common "casual" approach.
Even if you really have some really rare, intense job skills, high security clearance, etc. you need to look professional.
Especially when job hunting. You will see some of the company staff manning the tables dressed casually - but remember they will be there for a very long day(s) and sometimes will fall back on a more relaxed dress code. Managers, hiring managers, will more likely wear a tie.
On the job site collared polo style shirts and dress slacks were allowed for certain people. Usually the technicians/engineers who had to crawl underdesks, run cables through ceilings etc. That dress code could be and was relaxed for especially heavy work situations: e.g., relocating an office with large numbers of computers, printers, scanners, etc.. Probably not T-shirts, but possibly blue jeans... Then there were casual Fridays but there were still some rules such as no holes in jeans etc..
When in formal meetings with supervisors and customers a tie is appropriate. Dress does not make the person, however, the person makes the dress by acknowledging seriousness, hierarchy, and professionalism. (A manager who cannot/will not communicate does not make up for that via a $500 suit. Often can fool people though.)
Once you have been in an environment for a while the "proper attire" requirements become apparent and you adjust accordingly.
For interviews, a professional look and demeanor is key. To me that does not mean a high $ fitted suit. But it does not mean wearing navy blue slacks and a bright yellow blazer either. You need to be comfortable in the clothes that you are wearing. (Be sure to test wear everything, move and walk around for a while - hard to focus on questions, body language, etc. when something too tight is making you uncomfortable. Blazer/suit coat can prove to be too hot, shirt collar/tie too tight then you start sweating. Not good. Things are tense enough as they are without adding to the discomfort.
Yes some people are insistent about a suit and no problem with that per se. However, there is a bigger picture involved and dressing appropriate really should considered as expected without further ado. Hopefully those same people can provide more applicable and appropriate advice than just "you must wear a suit". Would not be surprised if they also insist on some introductory video (at extra cost to you) to electronically mail out to prospective employers.
It is always easier to take off a tie and/or blazer you have on than put on a tie and blazer you do not have..... Take some other clothes along to change into afterwards if viable. Will help you unwind on the way home. Maybe just the shirt and tie - switch to whatever makes you relax a bit.
For the the job fair and all interviews - dress and maintain the professional profile. And, by the way, turn off your mobile devices!