GET /categorias

curl    

 curl -X GET "https://bling.com.br/Api/v2/categorias/json/"
      -G
      -d "apikey={apikey}"

PHP

$apikey = "{apikey}";
$outputType = "json";
$url = 'https://bling.com.br/Api/v2/categorias/' . $outputType;
$retorno = executeGetCategories($url, $apikey);
echo $retorno;
function executeGetCategories($url, $apikey){
    $curl_handle = curl_init();
    curl_setopt($curl_handle, CURLOPT_URL, $url . '&apikey=' . $apikey);
    curl_setopt($curl_handle, CURLOPT_RETURNTRANSFER, TRUE);
    $response = curl_exec($curl_handle);
    curl_close($curl_handle);
    return $response;
}

C#

public static void ExecuteGetCategories() {
    var request = HttpWebRequest.Create(@"https://bling.com.br/Api/v2/categorias/json&apikey={apikey}");
    request.ContentType = "application/json";
    request.Method = "GET";
    using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
    {
        if (response.StatusCode != HttpStatusCode.OK)
            Console.Out.WriteLine("Error. Server returned status code: {0}", response.StatusCode);
        using (StreamReader reader = new StreamReader(response.GetResponseStream()))
        {
            var content = reader.ReadToEnd();
            if(string.IsNullOrWhiteSpace(content))
                Console.Out.WriteLine("Empty Response");
            else
                Console.Out.WriteLine("Response Body: \r\n {0}", content);
        }
    }
}