Does a side blog count as a business expense if it builds credibility for client work?

Running a small marketing business on the side. Most of my client work involves content strategy, but I also run my own blog and test ad campaigns as a way to learn and eventually monetize. Some of those experiments cost real money: ad spend, stock images, email tools, a domain or two. The question is how cleanly I can deduct that stuff when the blog itself hasn't made money yet but feeds directly into skills I sell to clients.

The part I keep going back and forth on is the intent question. I know the IRS cares about whether something is a real business or a hobby, and I know there are factors they weigh beyond just whether you turned a profit. My marketing business is profitable. The blog is an extension of that, not a separate thing in my head, but it looks separate on paper.

Does it make more sense to treat the blog costs as marketing expenses for the main business, since they build credibility and demonstrate expertise to potential clients? Or does running it as its own entity with its own potential revenue stream create more problems than it solves?

Also wondering what documentation people actually keep for this kind of gray area. Receipts are obvious, but is there anything else that helps establish the connection between the experimental spend and the primary business purpose if it ever got questioned?