I agree, there should be some sort of brand guide. You are right to question it. The problem Is that, often developers pitch themselves as web design specialists and somehow actual design falls by the wayside, which is why there are so many hing-functioning, ugly, ineffective sites out there. This may, or may not be the case here, but it is a common scenario, which is why clients got to web designers for one thing and actual designers for the rest.
I have had this happen a few times. The way I play it is go back to the client and say what you’ve said here. Explain to them that for your work to be as effective as it can, it needs to be consistent with their existing brand. Ask if they have any sort of manual (hint; they won’t) or alternatively ask for a contact at their web designers so you can collaborate.
You will learn pretty quickly whether their site is designed and developed or just ‘designed’ by developers. On a few occasions, once I have built a relationship with a client, if their site is developed and not designed, I have had a conversation with them and ended up working with their developers to make the site communicate effectively, not just function well
It’s all about trusted relationships with clients. Talk to them about your misgivings. At very least it will help build trust and they will know that you know what you are talking about, rather than just giving them what they want and invoicing taking the money. After all, how often have we all had clients come to us asking for one thing and after discussing it with them, you find what they want and what they need are two different things.