Translation:These pants are black.
True! It's used to put strong emphasis on words (or for 'cool factor'), as the relative simplicity of katakana really makes them stand out from the kanji & hiragana jumble in a sentence.
Also used for animal names, since their kanji are often way too complex even for Japanese (and technically loan words from Chinese). Like クモ（蜘蛛）'spider' , and コウモリ（蝙蝠）'bat'.
Short answer: katakana is used to make them stand out.
Normally you'd use kanji for nouns, but as said above, those are particularly complicated for animals, and they're kind of loanwords anyway.
The pronunciation is the so-called onyomi ('Chinese' reading), which dictionaries also tend to show in katakana (as opposed to the kunyomi (Japanese reading, shown in hiragana)). Plus, using hiragana might make them difficult to distinguish from surrounding verbs, particles, etc., as you may have noticed yourself already.
Apparently, in the past 蟲（that's 3x 虫）was used to refer to a whole bunch of small animals. This "critter" category included birds, fish, and even mammals. There were winged "critters", scaly "critters", hairy "critters", etc. So although we don't think of them as bugs today, many kept that radical once they got their own kanji.
Other examples -that I know of- are 蛙 (frog) and 蛇 (snake), but there are likely many more.
The fact that the "spoken" Japanese questions often don't accept common kanji makes it extremely difficult to learn optimally. Having to force myself to write, for example, くろい when I instinctively want to make it 黒い is unfortunate. The kanji is technically more correct, and it is accepted as an answer in the non-spoken questions.
zubon means pants/trousers. In some English speaking countries pants means underpants/underwear and in some English speaking countries pants mean trousers and sometimes it means both - in the places where pants can mean both trousers and underpants you'd probably still get some muffled sniggers if you used pants to mean trousers, just because of the double meaning. There's been some discussion on here about whether pantsu means pants/trousers or underwear, I still hold that pantsu=zubon. shitagi (literally - under clothes) is the word for underwear, although not necessarily a word that exclusively means underPANTS - shitagi, as I understand it, refers to all undergarments so could refer to underpants, bras, singlets.
Please, Duolingo. I'm begging you. Fix the listening section questions so that they (consistently) accept the proper kanji. I'm going crazy here. To make things even more frustrating, there is no "My answer should be correct" option in these questions to report them. For a few colours, the listening sections accepted the kanji. But then I hit this one, and it won't accept 黒い. Trying to type everything in hiragana is annoying and slow, and then sometimes, when you DO write it all in hiragana, it won't accept the answer without a particular kanji. I'm going up the wall, here.
In Japanese, one generally can't distinguish singular or plural by words alone; it's all up to the context. However, one can distinguish them if they are made more explicit by:
A) counters to indicate the exact number, such as 本（ほん）for pants (e.g. 一本（いっぽん）のズボン = 1 pair of pants/trousers) or 人（にん）for people (e.g. 四人（よにん）= four people). Various types of objects and animals have their own counter. See: https://en.wikipedia.org/wiki/Japanese_counter_word
B) modifiers attached to the word, such as ～達（たち）for people and animals, (e.g. 私たち = a rather emphatic "we", for example used to distinguish between yourselves and another group, or 鳥たち = "the birds". Again, used when being fairly specific about which ones).
It's not being inflexible. この and パンツ go together - these pants. この cannot be used independently - it MUST go with a noun - in this instance パンツ. What you have written would be これは くろい パンツ です. Notice how これ can be used independently and does not have to modify a noun? Also in your sentence くろい modifies パンツ rather than この.