Final answer:
The revised and trimmed sentence is 'The flock of birds darting over the lake was suddenly opalescent silver,' which is more concise and has correct verb agreement.
Step-by-step explanation:
To revise the sentence 'The flock of birds that darted over the lake were suddenly an opalescent silver', we need to trim it for conciseness and correct the verb agreement. The revised sentence could read: 'The flock of birds darting over the lake was suddenly opalescent silver.' This trimmed version is more direct and uses the singular verb 'was' to agree with the collective noun 'flock.'