On-site blog leading to decreasing sales?

Hey guys!
My first post in this forum

I run an ecommerce fashion website, and have been seeing some fluctuations in revenue from organic visitors. The traffic from organic sources is more or less stable (yes, I wish it was increasing, but I'll save that for another post), while it seams the sales peaked a few months ago, and have decreased.

I have read some things about internal "competition" on websites, as in when some pages outperform others in search engines. We run a blog on in our webshop which is generating organic traffic, but not really any revenue.

I am wondering if the performance of the blog articles can have a negative effect on the pages that actually generate revenue (product/landing pages)? Will search engines favour the pages that perform well in search engines (blog articles) giving them more display than the designated landing pages (product pages)?

Hope that makes sense, and that someone has some good insight on this