I'll fancy some educated guessing:
mean, black was completely black and no seperation between multiple black objects
Exposure problem, more light needed...
But on "lighter" appearing frames that were slightly out of focus there was no seperation issue with black on black..
...which appears you have given already, but by using a slower shutter speed to get more light, you introduced a slight motion blur.
In Macro photography the movement gets amplified just the same way as with shooting long focal lengths (max zoom), one must be aware of it.
There are easy ways to fix it:
- More light. Macro is hungry for light and photography is about nothing but light. More of it will allow you to use faster shutter speeds and have more details in focus by closing down the aperture.
- Use tripod to ensure minimum vibrations. As said before, motion gets amplified = becomes blurry fast. If tripod isn't available and you have a metric shitton of light, you can risk handheld. Just remember to use fastest shutter speed available from the available range.
- Use Mirror Lock-Up and Self-timer if available to reduce vibrations further. Tripod mandatory for this, of course.
As with all lenses, to reduce motion blur the old rule of thumb applies - set your shutter speed 2x the value of focal length. Meaning - if using a 70mm lens for macro, minimal shutter speed shouldn't be lower than 1/125 or equivalent. And with 200mm - no less than 1/500, ideally faster to freeze any motion.
Take this into consideration, reshoot and report your results! : )