Many CSVs to list, optimize
Since you only want lines starting with BLAH, filter the lines before parsing the CSV. I think the profiler is misleading you by attributing the time spent parsing CSV to the line for row in reader in your code.

with open(file, 'rt', newline='', encoding='utf-8') as f:
    filtered = (line for line in f if line.startswith('BLAH'))
    reader = csv.reader(filtered, delimiter='\t')
    for row in reader:
        files_found_lines.append(row[0:9])