how to remove Black background between start new activity during slide_left animation?

Setting the Theme didn't work for me, but adding an exit animation did.

overridePendingTransition (R.anim.push_up_in,R.anim.hold);

For the exit animation, I just used an animation that does nothing.

<?xml version="1.0" encoding="utf-8"?>
<set xmlns:android="http://schemas.android.com/apk/res/android">
    <translate android:fromYDelta="0%p" android:toYDelta="0%p" android:duration="2000"/>
</set>

All you really need, especially if you already got a theme set to the activity and don't want to use the Theme.Translucent suggested is add the following to your activity/app's theme:

<item name="android:windowIsTranslucent">true</item>

set the theme of that activity as transluscent in manifest file

android:theme="@android:style/Theme.Translucent"

so your code will be something like this

<activity android:name=".AdActivity"
        android:theme="@android:style/Theme.Translucent" />

If you are using the AppCompat ActionBarActivity you will need to use a theme that extends Theme.AppCompat

To give me the option to add background transparency to just the activities that needed it (ones launched using the intent flag_activity_new_task) but keep the background for the rest of the app.. I extended my main theme and set the transparent background options in that style.

<!-- The main theme applied to the application or activity -->
<style name="Theme.app" parent="Theme.AppCompat.NoActionBar">
    <!-- Your main app theme items go here-->
    <item name="android:windowBackground">@drawable/some_drawable</item>
</style>

<!-- Transparent background for app / activity -->
<style name="Theme.app.Translucent" parent="Theme.app">
    <item name="android:windowIsTranslucent">true</item>
    <item name="android:windowBackground">@android:color/transparent</item>
    <item name="android:colorBackgroundCacheHint">@null</item>
</style>