That depends on where you work and who you work for. A few employers actively encourage staff to discuss work on their blogs, but many are actively hostile. Some have no clear rules at all, and will only draw some up when they come across an employee blogging about them. If you're going to be that employee, you need to be prepared.
The first step to discovering your employer's position, before you write anything at all, is to check your contract and the organisation's policies to see if there are rules specifically covering blogging. A good policy will be clear about what you can’t put on your blog as far as work is concerned and, however harsh it may seem, you should stick to it.
If there is no specific blogging policy, there may well be one on communications and conduct generally, or on social media use, which is likely to cover most of the concerns employers have about blogging, such as making sure you don't disclose confidential information, protecting the organisation's reputation and use of the Internet at work.
If there are no written rules, and you are determined to write about your work, the best thing to do is to broach the issue openly with your employers, tell them what you would like to do and that you might like to talk about work and ask how you can make sure they're happy with that. They may respond by threatening you with dismissal if you say anything negative about work, or they may welcome the chance to set down some clear rules that help them engage with a popular activity. Either way, you'll know where you stand.