line1
%%[
    SET @test = 'bla bla'
]%%
line2 %%= ContentBlockByKey("testExisting_asset_htmlblock") =%% still line2 line3
<script runat="server">
    Platform.Function.ContentBlockByKey("testExisting_asset_htmlblock");
    Platform.Function.ContentBlockByKey("testExisting_htmlblock1");
</script>