Which blackberry? A few are indica dominate and a few are sativa dominate. Could mean the difference between a week 8 harvest or a week 11 harvest.
I hate amber so I tend to chop as soon as I see amber as long as there is mostly cloudy, but like bigv said trich colors are not all the factors, although for me it is the main. Are your fans mostly yellowing or at least light green? Are the white pistils mostly receded and not white anymore? Do some our your true leaves have a dark and/or purplish color? So many factors to decide. Just because you see a few amber trichs doesn't mean you will see more tomorrow or next week, amber can take many weeks to spread.
Without pictures I can only say if there is mostly cloudy and only 5-10% clear or half cloudy then I'd cut. It takes a few harvests to understand a strain. At 42 days from 12/12 I'd say wait another 2 weeks, if 42 days 'from signs of first white pistils' which is usually 10-14 days into flower I'd cut in a week. But it's all guessing being I'm not there.