The consultancy analysed the websites of the 100 companies and found that one in three did not even state what their business did on the homepage.
Only five per cent clearly articulated their strategy, business model and KPIs while less than 50 per cent were ‘responsive’, meaning they did not deliver a quality experience for mobile use.
The survey also revealed that the websites failed to provide adequate information to key stakeholder groups.
Almost two-thirds did not provide a case for investment in the Investor Relations section of their website, while less than half included a description on the positive reasons to work for the company in the careers section.
However, 80 per cent of the FTSE100 companies had a corporate social responsibility section in the main navigation of the website.
But Black Sun said that the content of these sections, in many cases, failed to provide clear insights into the approach, objectives and performance of their corporate responsibility.
Only 14 per cent had a fact sheet available for the media providing information such as an annual report.
Richard Dixon, chief digital officer at Black Sun, said: "This research shows many companies are failing to make the most of their websites, despite it being a first port of call for information. The website is one of the few channels through which a company can tell its corporate story and should be seen as an opportunity to engage and influence their diverse stakeholder communities, to deepen their understanding of the business and to build trust and confidence in their brand."