First make sure that those blank lines do not come from empty paragraphs. Enable View
>Formatting Marks
to have clues about the real structure of your document. Erase those empty paragraphs.
Inter-paragraph spacing is a property of paragraph style. Identify which paragraph styles are in effect in those paragraphs: when the cursor is in the paragraph, the leftmost textbox in the toolbar displays the style name. Open this style to modify it. Spacing is defined in Indents & Spacing
tab. Change above/belox spacing to your liking.
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