I wanted to submit a solution which allows you to reuse your function code, if you want. You can define the function elsewhere, and then call it from the HTML element.
Somewhere in the myView.cshtml file (maybe at the top of the body tags) you can define a script and define myFunc inside, like this:
<script>
function myFunc() {
alert('fired');
}
</script>
Then call it from the HTML element, like this:
<%=Html.TextBox(
"ChooseOptions.AddCount" + order.ID,
(order.Count > 0) ? AddCount.ToString() : "",
new { @class = "{number: true} small-input", onblur = "myFunc()" }
) %>
Or, if you're happy to use a HTML helper to define your textbox, you can do
@Html.TextBoxFor(m => m.ChooseOptions.AddCount,
(order.Count > 0) ? AddCount.ToString() : "",
new { @class = "{number: true} small-input", onblur = "myFunc()" }
)