Use a list instead and replace your foreach loop with a for loop:
@model IList<BlockedIPViewModel>
@using (Html.BeginForm())
{
@Html.AntiForgeryToken()
@for (var i = 0; i < Model.Count; i++)
{
<tr>
<td>
@Html.HiddenFor(x => x[i].IP)
@Html.CheckBoxFor(x => x[i].Checked)
</td>
<td>
@Html.DisplayFor(x => x[i].IP)
</td>
</tr>
}
<div>
<input type="submit" value="Unblock IPs" />
</div>
}
Alternatively you could use an editor template:
@model IEnumerable<BlockedIPViewModel>
@using (Html.BeginForm())
{
@Html.AntiForgeryToken()
@Html.EditorForModel()
<div>
<input type="submit" value="Unblock IPs" />
</div>
}
and then define the template ~/Views/Shared/EditorTemplates/BlockedIPViewModel.cshtml which will automatically be rendered for each element of the collection:
@model BlockedIPViewModel
<tr>
<td>
@Html.HiddenFor(x => x.IP)
@Html.CheckBoxFor(x => x.Checked)
</td>
<td>
@Html.DisplayFor(x => x.IP)
</td>
</tr>
The reason you were getting null in your controller is because you didn't respect the naming convention for your input fields that the default model binder expects to successfully bind to a list. I invite you to read the following article.
Once you have read it, look at the generated HTML (and more specifically the names of the input fields) with my example and yours. Then compare and you will understand why yours doesn't work.