Does Tessellator GUI Rendering Support Transparency?

Hello everyone. I am having an issue with some GUI rendering I am trying to perform in my mod. I want to add a full screen multiple frame effect, and it all works, aside from that the transparent parts of the images are black and obscure vision.

I don't actually know much about GUI code, so a lot of this code is straight from the part of Minecraft that renders the pumpkin overlay, with some changes to add the multiple frame effect: 

public class TimeSkipEffectGUI extends AbstractGui {
    public static final Minecraft mc = Minecraft.getInstance();

    public void renderEffect() {
        PlayerEntity player = mc.player;

        if (mc.world == null) return;
        if (player == null) return;
        Stand.getLazyOptional(player).ifPresent(props -> {
            if (props.getTimeSkipEffectTicker() > 0) {

                RenderSystem.disableDepthTest();
                RenderSystem.depthMask(false);
                RenderSystem.defaultBlendFunc();
                RenderSystem.color4f(1.0F, 1.0F, 1.0F, 1.0F);

                Minecraft.getInstance().getTextureManager().bindTexture(new ResourceLocation(DiamondIsUncraftable.MOD_ID, "textures/gui/timeskip/time_skip" + (16 - props.getTimeSkipEffectTicker()) + ".png"));

                Tessellator tessellator = Tessellator.getInstance();
                BufferBuilder bufferbuilder = tessellator.getBuffer();
                bufferbuilder.begin(7, DefaultVertexFormats.POSITION_TEX);
                bufferbuilder.pos(0.0D, mc.getMainWindow().getScaledHeight(), -90.0D).tex(0.0F, 1.0F).endVertex();
                bufferbuilder.pos(mc.getMainWindow().getScaledWidth(), mc.getMainWindow().getScaledHeight(), -90.0D).tex(1.0F, 1.0F).endVertex();
                bufferbuilder.pos(mc.getMainWindow().getScaledWidth(), 0.0D, -90.0D).tex(1.0F, 0.0F).endVertex();
                bufferbuilder.pos(0.0D, 0.0D, -90.0D).tex(0.0F, 0.0F).endVertex();
                tessellator.draw();
                RenderSystem.depthMask(false);
                RenderSystem.enableDepthTest();
                RenderSystem.enableAlphaTest();
                RenderSystem.color4f(1.0F, 1.0F, 1.0F, 1.0F);
            }
        });
    }
}

The images I am using are transparent in all other settings I have tested them in. I am wondering, do I need entirely new rendering code, or can this code be adapted with some setting changes? It confuses me how the pumpkin overlay texture has transparency in it and works yet these images don't.