Internet is provided by the landlord of the business park where I rent as part of my utilities. Can she block internet content?

It depends on your lease. It is likely not covered other than to say the landlord will "provide a connection". Unless your lease is clear you will need to rely on the long term performance by the landlord. Arguably this is a change in the terms of your occupancy so the landlord would need to give you the required notice. If it is important to your business I would have an honest discussion with your landlord and explain it to them. I represent several landlords and none of them want to lose a good tenant.

This appears to be a matter of contract law. Contracts may be written agreements, oral agreements, implied by the circumstances, or a combination of these. Your lease agreement, email correspondence, and conversations may all make up your contract. A promise to provide internet does not necessarily mean that it will be the best possible connection or service. Another issue may be copyright infringement. The landlord may be concerned about streaming of infringing works and its potential liability. Or it may be prohibitively expensive to do what you would like. You could ask the landlord for its reasons. If it is a cost issue, you might ask how much it would need to do what you want, and organize the tenants to help compensate the landlord.

You are asking a question that a Contracts and Agreements attorney should answer. You should retain such an attorney to review your contract and provide you with an analysis of that agreement. With that analysis and a good reason to talk to the landlord, discuss with the landlord why it is that you need some particular streaming sites. I am not sure what you meant by the landlord "has decided to block some streaming sites instead of updating their system." Is this a new issue that has recently arised?