I wanted a block-based API for UIAlertView on my current project, but didn't want to import a whole Pod/library just for that, so I wrote a quick and simple implementation myself. Licensed under the WTFPL license. (http://www.wtfpl.net).

UIAlertView+BlockSupport.h

//
//  UIAlertView+BlockSupport.h
//
//  Created by Luke Redpath on 08/08/2013.
//

#import <UIKit/UIKit.h>

/* Called when the user taps a button in the alert view or the alert
 * is cancelled.
 *
 * If the alert was cancelled, wasCancelled will be YES and tappedButtonIndex
 * will return the value of [UIAlertView cancelButtonIndex].
 */
typedef void (^LRAlertViewCompletionHandler)(UIAlertView *alertView, NSInteger tappedButtonIndex, BOOL wasCancelled);

@interface UIAlertView (BlockSupport)

- (void)showWithCompletionHandler:(LRAlertViewCompletionHandler)completionHandler;

@end

UIAlertView+BlockSupport.m

//
//  UIAlertView+BlockSupport.m
//
//  Created by Luke Redpath on 08/08/2013.
//  Copyright (c) 2013 Luke Redpath. All rights reserved.
//

#import "UIAlertView+BlockSupport.h"
#import <objc/runtime.h>

@interface LRAlertViewBlockDelegate : NSObject <UIAlertViewDelegate>
- (id)initWithAlertView:(UIAlertView *)alertView;
- (void)showWithCompletionHandler:(LRAlertViewCompletionHandler)completionHandler;
@end

#pragma mark -

@implementation UIAlertView (BlockSupport)

- (void)showWithCompletionHandler:(LRAlertViewCompletionHandler)completionHandler
{
  NSParameterAssert(completionHandler);
  CNAlertViewBlockDelegate *blockDelegate = [[LRAlertViewBlockDelegate alloc] initWithAlertView:self];
  [blockDelegate showWithCompletionHandler:completionHandler];
}

@end

#pragma mark -

@implementation LRAlertViewBlockDelegate {
  UIAlertView *_alertView;
  LRAlertViewCompletionHandler _completionHandler;
}

#define kLRAlertViewBlockDelegateRefKey "LRAlertViewBlockDelegate"

- (id)initWithAlertView:(UIAlertView *)alertView
{
  if ((self = [super init])) {
    _alertView = alertView;
    _alertView.delegate = self;
  }
  return self;
}

- (void)showWithCompletionHandler:(LRAlertViewCompletionHandler)completionHandler
{
  _completionHandler = [completionHandler copy];
  
  /* We need to keep a strong reference to self or it will just be deallocated immediately.
   *
   * This technically creates a retain cycle between the alert view and self which is OK,
   * as we will make sure we break this retain cycle manually when the alert view is dismissed.
   */
  objc_setAssociatedObject(_alertView, kLRAlertViewBlockDelegateRefKey, self, OBJC_ASSOCIATION_RETAIN);
  
  [_alertView show];
}

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
  _completionHandler(alertView, buttonIndex, NO);
}

- (void)alertViewCancel:(UIAlertView *)alertView
{
  _completionHandler(alertView, alertView.cancelButtonIndex, YES);
}

- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex
{
  /* The alert view will be deallocced shortly after this is called if nothing else is retaining
   * it, so now's a good time to break the retain cycle.
   */
  objc_setAssociatedObject(_alertView, kLRAlertViewBlockDelegateRefKey, nil, OBJC_ASSOCIATION_RETAIN);
}

@end